In the following passage some words have been deleted. Fill in the blanks with the help of the alternatives given. Select the most appropriate option for each number.
Despite all the research every one of us catches cold and most of us catch (1) ______ frequently. Our failure to control one of (2) ______ commonest of all ailments sometimes seems ridiculous. (3) ______ science regularly practises transplant surgery and (4) ______ rid whole countries of such killing (5) ______ as Typhus and the Plague. But the (6) ______ of common cold is unusually difficult and (7) ______ is yet to be done to solve it. It is known that a cold (8) ______ caused by one of the number (9) ______ viral infections that affect the lining of the (10) ______ and the other passages leading to the lungs but the confusing variety of viruses makes study and remedy very difficult.
Select the most appropriate option to fill in the blank no. 7.