超声检查至少在妊娠几周后才能较好地看到胎儿（）。
A.2周
B.4周
C.6周
D.8周
E.9周
参考答案：D
一个长方形的草坪周长是240米，长是70米，宽是______米．
With the help of modern technology,the farmers have got twice________as they did before. [ ]
A. as much corn
B. much corn as
C. corn much as
D. corn as much
