“The reason is” in Japanese
Short answer
The reason is in Japanese is “それと言うのも” (soretoiunomo). Full meaning: the reason is ..., it is because ....
Other ways to say “The reason is” in Japanese
何故なら
nazenara because, the reason is, if you want to know why
ので
node that being the case, because of ..., the reason is ..., given that ...
野手
note that being the case, because of ..., the reason is ..., given that ...
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