反射密度仪适合测量（）。
A、实地密度
B、图文重影
C、墨层厚度
D、油墨乳化
参考答案：A
---He spent as much time as he could __________ his spoken English.
--- Wish him good luck !
A．improve
B．improving
C．to improve
D．improved
（）
A．200Hz
B．300Hz
C．700Hz
D．1500Hz
（）