So difficult ___________ it to live in an English-speaking country that I determined to learn English well.
Most whales must come up for air after several minutes, _______ a few can stay under water for an hour or more.
The numbers that researchers get from the experiments can be _______, just like number puzzles.
_______ the house is empty, memories flood my mind of a time when it was filled with life.
By the time a child is five, he _______ hundreds of hours of television.
There is nothing wrong with using a credit card for purchases. It’s more _______ than carrying loads of cash.