Last month i had an opportunity ---- at the local hospital.为

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