have an opportunity to do与doing的区别?

have an opportunity to do与doing的区别?
Last month i had an opportunity ____ at the local hospital.
为什么上空填的是volunteering,而不用to volunteer

就是to volunteer,不是volunteering
再问： 我们老师这么说的，而且参考答案也是这样写的。 那请告诉我have an opportunity to do与doing的区别？
再答： 没有have an opportunity doing sth 只有have an opportunity to do sth have an opportunity of doing sth. have an opportunity for (doing) sth. 不过这里的不是have an opportunity to do结构，volunteering at the hospital是在一起的的，它逻辑上是修饰opportunity的。 开始看错了，不好意思
再问： 所以呢？那用哪个？
再答： volunteering