A:to
B:on
C:do
—Jack hasn't taken his piano lessons for a long time.
—He is considering _________ his piano course and spending more time on his study.
用光学显微镜的一个目镜,分别与4个不同的物镜组合来观察某一细胞装片。当成像清晰时,每一物镜与载玻片的距离如图1,图2是图1中a组合观察到的视野。下列说法正确的是( )