You can only be sure of you have at present; you cannot be sure of something ___ you might get in the future.
23.— It’s a long time _____ I saw you last.— Yes, and what a pity! It will be a long time ____ we see each other again.
22.______ in natural gas and oil, so this area is worth developing.
21.Here’s one of the best ways to protect your skin—_____ too much sunshine.
20.There used__________a post office at the corner of the street.
19.It is ________ who ____________ wrong.