A.are many viruses that   62   colds

B.are still so   63   understood

C.is more   64   than it appears

One would think that with so many people having colds and so many hours of productivity being lost scientists would know very well about the common cold.But the disease   65   at first, On in the last few years have researchers gained certain knowledge about the agents responsible for it.For one thing, there   66  .The fact that colds are caused by viruses suggests some means of prevention and treatment.Because the environmental and physiological circumstances for the development of colds   67   , there is little one can do to prevent them beyond avoiding persons with "new colds" or staying away from others when one has a new cold.