图示桁架中a杆的轴力Na大小为：（）
A.+P
B.-P
C.+P
D.-P
参考答案：B
“杞人忧天”中的“杞人”的错误在于不懂得
A．物质决定意识
B．意识可以正确地反映客观事物
C．认识反作用于实践
D．事物的运动变化是有规律的
They are_________ to eat Chinese food though they have been in America for years.
A．interested
B．please
C． content
D．fond
PP
PP