3
[ying back coming back] to lokyo for the last [past] few
3
[several] years, but no one [nobody] knows (exactly) why
4
[knows the (exact) reason / knows the (immediate) cause].
(2) For some reason (or other), the number of swallows
5
returning to Tokyo has been decreasing over [A for] the last
few years.
1 swallows は複数形だから less and less swallows は不可。
2 この文の「ツバメが来る」 とは 「ツバメが舞い戻ってくる」なので
return が最適。
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