— Yes, I've returned it.
A:already; already
B:yet; yet
C:already; yet
D:yet; already
(25×125)×(4×8)=____
在探究凸透镜成像规律的实验中,当烛焰、凸透镜、光屏处于如图所示的位置时,恰能在光屏上得到一个清晰的像.利用这一成像原理的光学仪器是