It was raining heavily. Little Tom felt cold, so he stood ______ to his mother.

A．close

B．closely

C．closed

D．closing

下列哪项可以既提高轴向分辨力，又提高横向分辨力（）

A.提高探头频率

B.减小声束直径

C.增大声束直径

D.缩短脉冲长度

E.以上均不对