Whenever I go to Beijing, ____ is quite frequent, I will go to visit the Great Wall.
Joan had often heard ____ said that Marley had no money.
Carl Linnaeus made great contributions to ________ plant species.
He observed a stranger _______ around the store at 12 p.m. last night.
Once ______ , the plan can’t be changed.
They asked me so many ______ questions that I got ______.