Recent surveys around the world have found some very sad and troubling news. Bees are not doing well. According to Time: Science & Space, "an estimated ten million beehives ... have been lost since 2006." What's worse, scientists are not sure why. Most beekeepers will tell you that bees tend to die in large numbers every spring. This is normal and necessary to keep a hive of bees healthy. But when the deaths add up to more than half the bees in an entire country, that is something altogether different and dire. Scientists have called this "colony-collapse disorder" or CCD. This name describes the effect, though, and not the cause of the die-out. Scientists simply do not know what is killing the bees. Many things make it hard for bees. Varroa mites move into a colony and weaken it. Further, these mites carry viruses that attack those that survive. In addition, a new bacterium was recently discovered that attacks bees intestinally and kills them. Beekeepers go to great lengths to protect bees from these invaders, and for many years have been able to control them. All these attackers and the methods used to combat them do stress the bees, however. Many feel the problem is not some new threat, but a decline in the habitat available to bees. Development of grasslands once home to wild flowers and food for bees has been universal. Pavement makes it easy to drive, but it replaces wild grasses and other food sources for bees. Perhaps, long before the mites and parasites moved in, humans took away enough of the bees' habitat to weaken their health. We plowed under a varied habitat and planted—in many cases—a single plant like beets. Then we sprayed all kinds of chemicals on the beets to keep other bugs away and in doing so also harmed bees. Now, to treat all the mites and other invaders, bee keepers use additional chemicals to kill the invaders. These chemicals also shorten the life of the bees. Have the use of chemicals, loss of habitat, and invading parasites made life impossible for bees? Experts continue to argue various theories, but few have any answers. Colony collapse disorder is perhaps one of the most mysterious problems modern science has tried to solve. Whether the problem is environmental or an unknown disease, we will all feel the effects if bees continue to decline. Bees affect almost every aspect of the human food chain. From the fruits, vegetables, and grains we eat, to the grasses and grains we feed our livestock, we depend on bees to keep us healthy. According to the Natural Resources Defense Council, bees account for 15 billion dollars in crops. Honey is a treasure for sure, but the real gold is in the strawberries, apples, peaches, and green beans we eat and need to stay healthy ourselves. All told, the outlook for bees is not good. The outlook for humans may not "bee" so good either.

Which line from the text best summarizes the topic of the article?

Surveys around the world have found some very sad and troubling news.

Bees are not doing well.

What's worse, scientists are not sure why.

Scientists simply do not know what is killing the bees.