GMAT Sentence Correction (SC) | Help needed in understanding modal auxiliary verbs

Hello,

Regarding MODAL AUXILIARY VERBS – PART II discussed in GMAT club grammar book, I've stuck on the answer for the one of the practice questions posted on Page 153. The question is based on used to scenario, as per the information present on the material, we have
1st rule: subject + used to + verb in simple form. . .(this is similar to would)
2nd rule: subject + be/get + used to + verb + ing...(this is to show that someone either is accustomed to something, or became accustomed to something)


I
...

---