题目

    根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。 Sharks have lived in the world's oceans for millions of years. A shark has an extremely good sense of smell. It can find small amounts of substances in the water, such as blood, body liquids and chemicals produced by animals. These powerful senses help them find their food.   51  Sharks grow slowly. About forty percent of the different kinds of sharks lay eggs.   52   Some sharks carry their young inside their bodies as humans do. Some sharks are not Your teacher will not be happy ________you finish your homework.A．although B. when C. unless D. because