1 . Who is __________ (blame) for the accident? (所给词的适当形式填空）

2 . Over the years, there ______ (be) a number of different techniques to help designers approach this important point. (所给词的正确形式填空)

3 . Leonardo da Vinci (1452-1519) is said_________________(buy) birds kept in cages in order to have the pleasure of setting them free. (所给词的适当形式填空)

4 . She admitted _______(have）)trouble ________(balance) her studies and playing basketball. (用单词适当形式填空)