塑料全冠最常用的适应证是（）
A.前牙永久性修复
B.后牙永久性修复
C.固定义齿的固位体
D.保护性或诊断性暂时修复
E.以上都可以
参考答案：D
Everything he _____ to his country.
A．had was contributed
B．had been contributed
C．had to be contributed
D．had contributed
A perfectly competitive firm will continue to increase output so long as which of the following conditions exists（）
A. Marginal revenue is greater than price.
B. Marginal revenue is positive.
C. Market price is greater than marginal cost.
