Which of the following pieces of equipment would do best job to help prevent external rotation of the client’s right leg postoperatively

A. Sandbags.
B. A high footboard.
C. A rubber air ring.
D. A metal bed cradl

照片上各组织间的密度差异称为

A．组织对比度
B．胶片对比度
C．照片对比度
D．物体对比度
E．X线对比度