While the soldier’s idea was interesting the system was too difficult to be .A. of practi

问题补充：

While the soldier’s idea was interesting, the system was too difficult to be .

A. of practical use

B. practical usefully

C. practice use

D. practice used

答案：

(答案→)A. of practical use

B. practical usefully

C. practice use

D. practice used

解析：考查use的形容词性的用法，根据句子结构可知此处填的应当是形容词，或者名词，即practically useful,= of practical use