Last month i had an opportunity ____ at the local hospital.为什么上空填的是

have an opportunity to do sth
have an opportunity doing sth 两个搭配都是对的,但是意思有细微区别：
have an opportunity to do sth 有机会去做谋事（表将来）
have an opportunity doing sth 有机会做了谋事（表已经发生（
由于是上个月的事情,因此选 volunteering