---- you read through the letter,you will understand everyth

选A
if only仅仅是if的一种强化形式,不仅能表达说话人的愿望,而且能表达条件.
句意就是：假如你读过那封信,你什么都明白了.
例句：If only they were here now,we would be able to celebrate their wedding anniversary.如果他们现在在这里,我们就能庆祝他们的结婚纪念日了