_______, when Marx was already _______, he found it was important to study the situation in Russia, so he began to learn Russian.
The earth(地球) is our home, and we must _____ it clean.
–What would you like to drink? --______, please.
The children are playing a game _______ "hide and seek".
Today is Halloween. I ____ as a ghost, and of course I _____ a special costume.
There are _______minutes in _______hour.