The first and most important reason is that.....
    第一個(gè)而且最重要的原因是______________。
    One of the reasons that are given for....is that....
    ______________的原因之一是__________________。
    Perhaps this is because of the simple fact that.....
    也許，原因很簡(jiǎn)單，亦即_____________________。
    One of the premier causes is that(注意用詞)
    最重要的原因之一便是______________。
    At the risk of sounding too simplistic, it seems to me that the main propositions can be summarized in one saying.
    這個(gè)原因可能較簡(jiǎn)單，不過(guò)，我認(rèn)為可以用一句諺語(yǔ)來(lái)概括，________。
    We may look into every possible reason, however, foremost reason for .... Is ....
    其實(shí)有很多原因，不過(guò)，造成________的最重要的原因是______________。
    It might also be noted that....
    需要注意的是，____________。