It's a sentence from my dictionary:
Years of smoking have taken their toll on his health.
I'm wondering why it uses "have" instead of "has". I know "years" is in the plural, but "years of smoking" is just a single thing at large. The point is on "smoke", not "years". I can just simplify the subject as "smoking" if I want. But it would be wrong if I simplify the subject as "years" and omit "smoking" because time is not the reason, smoking is. So in this case, I feel the following sentence is right:
Years of smoking has taken their toll on his health.
But the right verb should be in the plural, why? How to explain this?