Thank you, but I’ll have to _______ your offer.
_____ he has done it is surprising. _____ he has done is surprising.
Here is such a difficult text _____ none of us can understand. Here is such a difficult text _____ none of us can understand it.
—Which of these two dictionaries will you buy? —I'll buy _____ ,for the different uses.
I didn't know _____ you were getting on well with your work.
Mr Black _____ his wife to teach their child a lesson. So he did.