孕几周是胎儿股骨长度的最易测量时间（）。
A.9～12周
B.14～22周
C.24～28周
D.32～36周
E.37～40周
参考答案：B
动物在生态系统中被称为______．
That's the new machine ______ parts are too small to be seen.[ ]
A. that
B. which
C. whose
D. what
