I walked slowly through the market,where people（ ）all kinds

I walked slowly through the market,where people（ ）all kinds of fruits and vegetables.I studied the prices carefully and bought what I needed.
A.sell B.were selling C.had sold D.have sold
此题正确答案是选B,此题强调的是 “我正在走着,看到了人们正在卖这些东西” 但是我不明白这句话用的为什么是过去进行时,而不是正在进行时?
那为什么不可以用”过去完成时“或”一般过去时“？