大兴林场去年栽松树386棵，栽的杨树是松树的3倍．栽杨树大约多少棵？
386×3≈1200（棵）．
答：栽杨树大约1200棵．
If they had known what was coming next, they __ their mind.
A．may change
B．could change
C．must have changed
D．might have changed
中央信号由（）组成。
A、事故信号
B、位置信号
C、预告信号
D、指挥信号
