中級英文文法問題

請教一下

We rented out the upper floor to outpatients over the years, so I had become accustomed to people with varied ailments ____ on our doorstep

a. appearing
b. appeared
c. to appear

請問哪個是正確的呢???
能說明為什麼嗎???
為何不能選b, c呢???


祝大家新年快樂 ^^
3 answers 3