The first and most important reason is that.....
    第一個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.
    這個原因可能較簡單，不過，我認為可以用一句諺語來概括，________。
    We may look into every possible reason, however, foremost reason for .... Is ....
    其實有很多原因，不過，造成________的最重要的原因是______________。
    It might also be noted that....
    需要注意的是，____________。