`Ans`:

`II. `

`19` Skytrans may be too expensive for students to travel.

`->` S + may be + Adj/N/V-ing

`-`too + adjective + for someone/something + to infinitive. 

`20` People should look right and left before crossing the road.

`->` should +v_bare

`21` Now many people are using fossil fuels, making them depleted.

`->` s+tobe+ving

`-` dhnb: now

`22` Did Anna help her grandmother with her shopping some minutes ago?

`->`Did + S + V (nguyên thể)?

`-` DHNB: some minutes ago

`III`. 

`23` Solar energy is a green, clean, and renewable energy source.

`->` s+tobe +n/adj

`-` adj +n

`24` Bamboo-copters are environmentally friendly. They will not cause pollution.

`->`  s+tobe +n/adj

`-` adj +n