The sentence "Hundreds of the vast blocks had been built before anyone began to doubt abou

t whether they were good solutions or not" means that ______.

A．it was not until hundreds of the vast blocks had been built that people began to doubt whether they were good solutions or not

B．hundreds of the vast blocks had not been built until anyone began to doubt whether they were good solutions or not

C．people began to doubt whether they were good solutions or not before hundreds of the vast blocks had been built

D．since hundreds of the vast blocks had been built, some people began to doubt whether they were good solutions or not