1. B. changing -> to change (decide +to V...)
2. D. to -> than (would rather sth than sth)
3. B. could you -> if you could (reported speech in request/ command: S1+ asked+O/ wanted to know+ if/whether+ S2+ V(one tense back))
4. D. help -> to help (ask sb for do sth)
5. D. knew nothing -> were known nothing (theo passive voice)
$\text{~ Chúc bạn học tốt ~}$
$\text{~ Nếu được thì cho mk trả lời hay nhất nha! ~}$