it's helpful with friends该填哪个 A to studying B studying C to study D study
问题描述：
it's helpful with friends该填哪个 A to studying B studying C to study D study
答
该问题涉及到 “形容词后加动名词或不定式规律“的语法：
例如：
第一类：表语是busy、worth等之类形容词,如下：（这些词比较特殊,日常碰到就总结起来）
be busy doing sth
be worth doing sth
.
第二类：表语是 important, necessary; essential; indispensable, meaningful等其他形容词,通常用不定式作主语表示一次具体的行为.如下：
be important to do sth
be necessary to do sth
.
形容词helpful属于第二类,所以选择答案为：C
希望能对您有帮助!