Idiom: One Swallow Does Not Make A Summer

This week’s idiom is summer-related, but what does it mean?

“You did well on your half-term test but one swallow does not make a summer.”

a) It wasn’t as good as you had thought.
b) Just because one good thing happens, it doesn’t mean more good things will happen.
c) You can feel confident from now on.
d) You did well, but really you should have done better.