For our middle school students, some courses are compulsory, while others are ________.

The shop-assistant was straight with his customers. If an article was of _____ quality, he'd tell them so.

The water of the lake was so __________ that it looked like glass.

 －Derrick, don't you think you should take a vacation? Even one or two days would be fine.－There's           . There's too much work.

In that school, English is compulsory for all students, but French and Russia are ____ .