超声造影回声强度减低的原因是

A、微颗粒减小 

B、发射超声强度增大 

C、检测深度增大 

D、微气泡截面积增大 

E、发射超声频率减低 

根据汉语提示完成句子

1. When the teacher came in, all the students                         (变得沉默了).

2. He                         (睡着了)on the sofa.

3. They                                                               (陷入爱河)when they met for the first time.