What some people don’t realize is ________ difficult life can be after they get addicted to drugs.

Scientists ______ SARS for several monthsbefore a major discovery was made in the field.

.As long as we are united, there will be nodifficulty in the world that we can’t__________.

Mark is very similar _____me, but he is a little different ______ me.

These toys are not suitable for children _____ four years old, who are too young[ ]