he felt very sad BECAUSE he had failed to pass the exam.
He felt very sad because of his failure to pass the exam.------ because of + 短语结构.
答案只能是BECAUSE,因为because重在强调“原因”,as的话,表“原因”的句子要放在句子的开头.
SO是这样的：
He had failed to pass the exam SO he felt very sad.
== He felt very sad BECAUSE he had failed to pass the exam.