A:转动转换器﹣﹣换用不同的物镜
B:用较大的光圈﹣﹣看到更大的视野
C:移动玻片标本﹣﹣找到要观察的物像
D:微调细准焦螺旋﹣﹣使物像更清晰
如图,在平面直角坐标系中,△ABC的三个顶点坐标分别为A(﹣2,1),B(﹣1,4),C(﹣3,2).
如图,∠A是⊙O的圆周角,若∠A=40°,则∠OBC=____ 度。
An ant was drinking by a small river when it fell in.It made ______ efforts to reach the side,but made no progress at all.
— About twenty minutes' ride.