—My son, who is now studying at a university, uses more than 3,000 yuan a month.

—Well, tell him to be economical. After all, ________.[ ]

A. money doesn't grow on trees

B. the morning sun never lasts a day

C. light come, light go

D. penny wise, pound foolish

Don’t talk such things ___ you don't understand.

A．that

B．which

C．what

D．as