✔ 最佳答案
因為該句是從下面這句演變來的：Some chemicals which are used for makeup were deadly to our body.
有些「被」用來做成化粧品的化學物，很傷害我們的身體。 在關代的句型中，which + be動詞，常可被省略，就成為：
Some chemicals used for makeup were deadly to our body.(意思不變)
所以，其它三個答案，均不適用。
2011-11-23 13:30:24 補充：
你若要用 “are used” ，那麼整句就要改寫成：
Some chemicals are used for makeup, and they are deadly to our body. (現在式：不變的真理、事實；其實以原句的意思用現在式會更合宜)。
Some chemicals were used for makeup, and they were deadly to our body. (過去式：發生在過去某時間點；而現在可能就不得而知是否還有這種狀況)。
they指“Some chemicals”。
2011-11-23 14:01:09 補充：
你若要用 “are used” ，那麼整句就要改寫成：
Some chemicals are used for makeup, and they are deadly to our body. (現在式：不變的真理、事實；其實以原句的意思用現在式會更合宜)。
Some chemicals were used for makeup, and they were deadly to our body. (過去式：發生在過去某時間點；而現在可能就不得而知是否還有這種狀況)。
they指“Some chemicals”。
