下列哪个不是孕激素的化学结构特点（）
A.具孕甾烷母核
B.A环为芳香环
C.3位和20位有酮
D.4位有双键
E.10和13位有甲基
参考答案：B
No one in the factory but the brothers _____ that our factory will be closed because of the financial problem.
A．knows
B．know
C．have known
D．knew
机床上进给螺杆，它的牙形角一般为（）。
A.30°
B.40°
C.60°
