He was so deep in thought,thinking of the problem ________ he knocked into the tree without noticing it.
--Did a letter come for me? --Yes, ___ came for you this morning.
He is determined to devote himself to ___ the country.
Whenever we were ___, he is always ready to ___ us some help.
The way ___ you are doing it is completely crazy.
The potato is a plant ____ in places ____ it is too cold to grow some other plants.