$#DUCK$
Don't spend too much time _____ made in the previous test. It's a waste of time.
A. discussing with the pupils what errors were
B. on discussing with the pupils about errors
C. discussing the errors
D. to discuss about the errors
$\text{Đáp án C là đúng vì Spend time/money (on) doing sth: dành thời gian/tiền bạc cho việc gì}$
$\text{Đáp án A sai về nghĩa, đáp án B, D sai về ngữ pháp -> Chọn đáp án C}$