a multiple-choice question
The ancient city at one time must ( ) prosperous, for It enjoyed a high level of civilization. Each of these represented a goddess and had,at one time ,been painted.
A. be
B. been
C. have been
D. had been

i think C and D would work. but I'm not convinced whether my choices is correct or not.
which one is correct? and why? Do the have or had been in here has any difference?

Question

a multiple-choice question
The ancient city at one time must ( ) prosperous, for It enjoyed a high level of civilization. Each of these represented a goddess and had,at one time ,been painted.
A. be
B. been
C. have been
D. had been

i think C and D would work. but I'm not convinced whether my choices is correct or not. 
which one is correct? and why? Do the have or had been in here has any difference?

Answer

Hi Nelson Chan,
The answer is "C". 
Don't use "must had been". Why? Because it doesn't exist. This is not a case of context, you can NEVER say "must had been".
After modal auxiliary verbs like must, we use the infinitive form of the verb (without "to"). But "had" is the past tense/participle form, so it's incorrect to use it.