单选题：根据以下材料，回答题What's Killing the BatsFirst it was bees. Now it i

根据以下材料，回答题
What's Killing the Bats
First it was bees. Now it is bats. Biologists in America are working hard to discover the cause of the mysterious deaths of tens of thousands of bats in the northeastern part of the country. Most of the bats affected are the common little brown bats (Myotis lucifugus), but other species such as the long-eared bat, the small-footed hat, the eastern pipistrelle, and the Indiana bat have also been affected. In some caves, more than 90 percent of the bat populations have died.
One possibility is disease. A white fungus (真菌) known as fusarium has been found on the noses of both living and dead bats. However, scientists don't know If the fungus is the primary cause of death, a secondary cause of death, or not a cause at all, but the result of some other conditions.
Another possible cause is a lack of food. For example, bats typically eat a large number ofmoths(蛾 ), and in some states such as New York, the number of moths has been declining in recent years. If bats can't eat enough food, they starve to death.
Still other scientists believe that global warming is to blame. Warmer temperatures in recent years have been waking up hibernating ( 冬眠 ) bats earlier than usual. If bats break their hibernation at the wrong time, they might not find their expected food sources. The weather might also turn cold again and weaken or kill the bats.
Scientists might not agree on the causes of the bat die-off, but they do agree on the consequences. Bats are an important predator of mosquitoes; a single brown bat can eat 1,000 or more insects in an hour. They also eat beetles and other insects that damage plant crops. If there aren't enough bats, damage will be great from the insects they eat While bats live a long time for their size--the little brown bat can live for more than 30 years--a female bat has only one baby per year, so bat populations grow slowly. Many bat species in the United States are already protected or endangered.
How can you help? Do not disturb sleeping or nesting bats. If you discover bate that seem to be sick or that are dead, contact your local Fish& Wildlife Department with the details. However,be careful not to touch the animals.
What is the main idea of this passage? A．All species of bats in North America are dying.
B．Scientists already know the cause of the deaths of bats.
C．The bat deaths are a serious problem.
D．There are many possible causes of the deaths of bats.