It's five years since my dear aunt _____ here. [ ]

A. left

B. has left

C. is left

D. had left

水平角观测中，为减少度盘分划误差的影响，应使各测回（）。

A.同一方向盘左、盘右读数取平均值

B.变换度盘位置

C.增加测回数

D.更换仪器