Fill in the blank with 1 word

I/ Fill in the blank with 1 word:
1/ The maximum class size is twelve ... the average is ten;
2/ You will take short ... in english as soon as u arrive;
3/ In this way, we can put u in a ... at the most suitable level;
4/ Ưe help u to use the english u have previously ... in your own country
II/ Write the correct form of verb:
1/ The classroom is still very dirty.I don't think it...(clean);
2/ After...(shout) for help, she...(take) off her shoes and... (jump) in... (save) him ;
3/ ...(write) the letter, she put it in an envelope
III/ Choose the wrong word and correct the mistake:
"Because of the high birthrate and considered immigration, the US population in the late nineteenth century increased tremendously from 31 million in 1860 to 76 million in 1900 "
The wrong word: A. birthrate, B.considered, C.in, D.increased