飞车走壁的演员不会掉下来的原因是 ( )
A．离心力的作用
B．演员们勤学苦练的结果
C．他们的身体被悬空绑着
D．魔术表演
参考答案：A
There are still many problems ______ before we are ready for a long stay on the Moon.
A．solving
B．solved
C．being solved
D．to be solved
命题“∃(x,y),x,y∈R,2x+3y+3<0”的否定是( )
A．∃(x,y),x,y∈R,2x+3y+3>0
B．∃(x,y),x,y∈R,2x+3y+3≥0
C．∀(x,y),x,y∈R,2x+3y+3≥0
D．∀(x,y),x,y∈R,2x+3y+3>0
