90°脉冲激发后，磁共振信号以指数曲线衰减，称为（ ）After 90 ° RF pulse excitation, the MRI signal attenuates exponentially is called（ ）
选项：

A:纵向弛豫Longitudinal relaxation;
B:横向弛豫Transverse relaxation;
C:自由感应衰减(FID)Free induction decay (FID);
D:自旋Spin