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.