能引起框移突变的有（）和（）突变。
参考答案：缺失；插入
Elephants are very friendly and they like grass. [ ]
A. eat
B. to eating
C. to eat
D. ate
机车进入挂车线后应注意，距脱轨器、防护信号、车列前（）m左右必须停车。
A.5
B.10
C.15
D.20
