My brother _____  to the ground while ____  his bicycle and hurt his leg .

A． falls; is he riding

B． fell; he was riding

C． fall; he is riding

D． felt; was he riding

关于X线照片颗粒度的叙述，错误的是

A．物理测定值为颗粒度

B．X线量了斑点影响照片颗粒性

C．X线照片上颗粒聚集的区域，称噪声

D．增感屏斑点是造成照片斑点的主要因素

E．量子斑点是X线量子的统计涨落在照片上的记录