The words in each set (1-7) frequently occur with ONE other word. What is it?

1 __________________
best, close, closest, dear, good, great, lifelong, old, trusted

2 __________________
clear, deep, firm, flat, gentle, gruff, harsh, high, hoarse, husky, loud, low, quiet, shaky, shrill, soft, strident

3 __________________
arduous, epic, gruelling, hazardous, long, short

4 __________________
bad, fatal, horrific, minor, nasty, serious, tragic

5 __________________
dead-end, full-time, good, low-paid, menial, part-time, permanent, proper, semi-skilled, skilled, temporary, unskilled, well-paid

6 __________________
correspondence, intensive, introductory, refresher, residential, vocational

7 __________________
bend, break, flout, follow, obey, play by, relax, stick to, stretch