— 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

有一种游标卡尺，主尺的分度值是1㎜，游标卡尺上有50个等分刻度，此游标卡尺的总长度为49㎜，用这种游标尺测长度可以准确到         ㎜，用此卡尺测一木球直径时，游标卡尺刻线是在3.2㎝和3.3㎝两刻线之间，若游标卡尺的第19条刻线与主尺刻线对齐，则此木球的直径为               ㎝。