Less is known about the cause of the common cold than about the causes of many more serious diseases.
A.The causes of less serious diseases than the common cold are better known than it is.
B.We know less than we should about the causes of the common cold and more serious diseases.
C.We know less about the cause of the common cold than we do about the causes of more serious diseases.
D.The cause of the common cold is better known than the causes of more serious diseases.

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