21. Do you know the ______ of the saying I just quoted?

A. source           B. resource            C. course          D. cause

22. Generally speaking ______according to the directions, the drug has no side effect.

A. when taking       B. when taken.        C. when to take.     D. when to be taken.

23. Helen always helps her mother even though going to school ______most of her day .

A. takes up.         B. makes up.          C. saves up.        D. puts up.