The bus is (quick quickly),but it isn't(quiet quietly)

quick,quiet.这两个都得填形容词,因为前面是be动词,形成主系表结构.quickly,quietly都是副词,是修饰实义动词的.
再问： 那么 Alice usually drives(safe safely),but last night she was(careless carelessly)
再答： 第一个是safely，副词修饰动词drive，第二个careless，又是主系表结构，填形容词。