John's success has nothing to do with good luck. It is years of hard work _____ has made him what he is today. [ ]

A. why

B. when

C. which

D. that

自然组织频移谐波成像的作用是（）

A.增加图像可视帧频 

B.增加界面分辨率及清晰度 

C.提高超声输出功率 

D.增加高频超声的穿透深度 

E.可以减少超声的伪像