optional

柯林斯詞典

1. ADJ If something is optional, you can choose whether or not you do it or have it. 可選擇的
Sex education is a sensitive area for some parents, and thus it should remain optional. 性教育對(duì)一些家長(zhǎng)來(lái)說(shuō)是個(gè)敏感的話(huà)題，因此應(yīng)該還是非強(qiáng)制性的。

Seat-belts are included as standard but electric windows are an optional extra .
My phone came with 20 optional ringtones .
Do come to the party - fancy dress is optional.
There are several optional tours , for which there will be a small charge .
The basic model is quite cheap , but there are a lot of optional extras .