conclusion: all right
a great quantity of flowers was/were placed in the hall.
1 'a' stands for 'one', we wouldn't say "one of the men are"...
2 but we do say "a large number of books are interesting.". the "number of" takes plural verb in many cases.
so, I thought whether to use a singular or plural was decided by what the point you wanna emphasize ... id est: "a quantity" or "flowers"

是正确答案吗

都flowers了还was

额，were吧，你换成a lot of就行了，除非a quantity of就是这个用法

a great quantity of = a large quantity of 意思是"大量/许多"。可以修饰可数名词复数也可以修饰不可数名词。接可数名词复数作主语时应该视之为复数，谓语应该用复数，这里应该用were。严格地说用was是错误的，但现在口语会话中也常常有人用单数was。

这里用was,因为形容a quantity , 不是后面的flowers

each everyone 这些都是s

分清真正的主语是谁了么？是quantity！