问题详情

According to the passage, why couldn"t PET and functional M. R. I. scans detect subtle changes in blood flow earlier？
A．Because there is early oxygen transfer.
B．Because they do not form. their images quickly enough to follow such rapid changes.
C．Because researchers control the changes in blood flow.
D．Because early oxygen transfer gives the flow blood energy.请帮忙给出正确答案和分析，谢谢！