Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 28 to 35.

The blue whale is the largest animal ever known to have existed. During the 20th century, the species was almost exterminated due to commercial whaling. The species has slowly recovered following the global whaling ban but it remains endangered and faces a number of serious threats including ship strikes and the impact of climate change.

Blue whales are simply enormous with most ranging in length from 24-30 m. The largest ever recorded was a gargantuan 33.5 m long. Females are up to 10 m longer than males. And they can weigh up to 200 tonnes. Just to put that in perspective: an adult male African elephant weighs 6 tonnes. The blue whale's heart is the size of a small car and its beat can be detected two miles away. But that's nothing compared to their calls. Blue whales are the loudest animals on earth and their calls are louder than a jet engine: reaching 188 decibels, while a jet's engine hit 'just' 140 decibels. Apart from their gigantic size, blue whales can be identified by their relatively small dorsal fin, a fairly rounded rostrum (anterior part of the skull), and approximately 90 ventral grooves, which reach the navel. They also have row of 300-400 baleen plates on each side of the mouth, which are black in color and range in length from 50 cm in front to 100 cm in back.

Blue whales mostly travel alone or in groups of 2-3. Larger groups of up to 60 whales have been reported and are probably associated with feeding grounds. However, the blue whale has the most powerful voice in the animal kingdom and its low-frequency sounds can travel in deep water over hundreds, or even thousands, of miles. Under these circumstances, animals which may appear to us to be traveling alone may actually be in constant contact with one another.

At birth, a blue whale calf is the largest baby on earth: approximately 8m long and weighing about 4 tonnes. They grow at a rate of 90 kg per day and wean after 7-8 months, once they have reached about 15 m in length, and are able to follow the normal migration pattern alone. They reach sexual maturity at 5-10 years. This growth rate is astonishing and is probably the fastest in the animal kingdom. From conception to weaning, it represents a several billion-fold increase in tissue in just over a year and a half. Like other baleen whales, the blue whale has no teeth so it is hard to tell its age but scientists believe they live until at least 50.

Like other large whales, blue whales are threatened by chemical and sound pollution, habitat loss, overfishing of krill, ship strikes and becoming entangled in fishing gear. Climate change could also have a major impact on its food supply, since global warming and associated ocean acidification may impact krill populations. In addition, frontal zones - critical whale habitats - are projected to move further south due to climate change. Frontal zones are boundaries between different water masses, where water can rise from the depths, bringing with it large amounts of nutrients that stimulate the growth of phytoplankton and support substantial populations of prey species for whales. Blue whales would have to migrate further (perhaps 200-500 km more) to reach and feed at these food-rich areas where they build up reserves to sustain themselves for the rest of the year. These longer migration paths could increase the energy costs of migration and reduce the duration of the main feeding season. As frontal zones move southward, they also move closer together, reducing the overall area of foraging habitat available.

What does the word "wean" in paragraph 4 mean?