适宜的前池底坡坡度i应在（）的范围内选取。
A.1～2
B.3～4
C.0.2～0.3
参考答案：C
着床前的胚胎易发生（）
A.生长迟缓
B.受精卵死亡
C.致畸作用
D.母体毒性
E.母体死亡
My mother _________ a teacher for ten years. [ ]
A. has has
B. has been
C. have had
D. has had
