— Tom says he is too tired to go on.
— He can't. He ________ for an hour.
A:had worked
B:has just worked
C:just worked
D:has only been working
将点M(2,﹣3)关于y轴的对称点向上平移4个单位长度到M′,则点M′的坐标是____.
(Ⅰ)求椭圆C的方程;
(Ⅱ)求 OA→ • OB→ 取值范围;
(Ⅲ)若B关于x轴的对称点是E,证明:直线AE与x轴相交于定点.
台湾发展农业生产有利的自然条件是 ( )
平面上有6条两两不平行的直线,求证:在所有的交角中,至少有一个角小于30.1°.