I encountered a question at the end of a unit about how to use “think about” and “think of”. It’s stated that we use “think about” when we consider an idea or concentrate on it, and we use “think of” when an idea comes into our mind. The question is a fill-in question:
“I don't really want to go out with Tom tonight. I'll have to … an excuse.”
I think we have to put “think about”, because we have to consider or concentrate on an excuse. But in the answer section, “think of” is stated as the answer. Which one is grammatically correct? Isn’t “think of” wrong? Why?