---I’m sorry that I didn’t work out this problem. ---It’s _____our ability. I didn’t work it out, too.
The _____ stuck on the outside of the envelope said "By Air".
It was the largest experiment we had ever had; it _____ six hours.
I have been looking for this book for months, and _____ I have found it.
He offered to _____ her a hand as the suitcase was too heavy for her to carry.
I'm sorry but what he thinks is not of the _____ importance to me.