Bob would have helped us yesterday, but he-_______

A．was busy

B．is busy

C．had been busy.

D．will be busy

关于MRI血流常见形式层流的表述，正确的是（）

A.层流血流越靠近血管腔中心的血流越快

B.层流血流越靠近血管壁的血流速度越快

C.层流在血管内的流速呈杯口状

D.层流在血管内的流速呈抛物线状

E.层流在血管腔中心的血流最快