下列有关乳腺结构不良的X线表现中，描述不妥的是（）

A.乳腺实质内多发或弥漫性不规则结节影

B.病变严重者常难分辨正常乳腺腺体结构

C.有时可在其内见纤维条索影，边缘模糊

D.乳腺实质内无钙化影

E.少数患者乳腺内可见新月形钙化影

It is not until recently ______ the scientists have found proof of there being water on Mars.

A．when

B．that

C．how

D．what