21.

1. timely
2. trimmed
3. tiny
4. topical

22.

1. sky
2. atmosphere
3. oxygen
4. environment

23.

1. only
2. too
3. that
4. extremely

24.

1. construct
2. structure
3. piece
4. feature

25.

1. but
2. also
3. thus
4. nevertheless

26.

1. putting
2. pairing
3. placing
4. preparing

27.

1. while
2. when
3. therefore
4. also

28.

1. piece
2. type
3. atom
4. measure

29.

1. from
2. in
3. to
4. for

30.

1. different
2. usual
3. important
4. chemical

(Reference keys.)