题目

Exactly _________ the potato was introduced into Europe is uncertain, but it was probably around 1565. A . whether B . when C . why D . how 答案：B某同学用显微镜观察时，若光线很强，则选用的光圈和反光镜依次是A、较小的光圈，凹面镜 B、较大的光圈，凹面镜C、较大的光圈，平面镜 D、较小的光圈，平面镜