—I'm afraid we can't work out the math problem________the teacher helps us.
—That's true. It's too difficult.
句意：——如果老师不帮助我们，恐怕我们解不出这道数学题。——那是真的。它太难了。
考查连词辨析。if如果；是否；unless除非，如果不；because因为；as soon as一……就。根据句意“It’s too difficult”可知，这个数学题很难，因此这里表示“如果没有老师帮助，就不会……”，应用unless引导条件状语从句。故选B。
—I'm afraid we can't work out the math problem________the teacher helps us.
—That's true. It's too difficult.
I don’t like Sun Cinema ________ it has bad service.
Check what you have written _______ you hand in your application form.
We still know little about the Moon _____ men have landed on it.
The movie Nezha is_______ educational ____ I want to see it again.
You can surf the Internet ______ you want to know more about the 2020 World Middle School Games.
—How time flies! The middle school life is coming to an end.
—Yes. We have to say goodbye, ________ our friendship will last forever.
Mike and Jim's room is tidy, ______ their sister's not.
Our English teacher is nice and patient she is very strict with us.
Even if we learn something well, we will forget it ______ we use it.
