あさっては大雨【おおあめ】が降【ふ】るそうだよ。ハイキングは来週【らいしゅう】（ ）。(source: pg. 55, 新完全マスター)
The choices available are:
According to the textbook, the correct answer is b, though I am confused as to why a is invalid.
As far as I understand, a would translate to something like "let's try to go next week", where as b would translate to "let's decide to go next week".
Both choices seem to make sense, and I am seeking an explanation as to why b is preferable over a, and furthermore, perhaps a more general rule for determining which construct to use.
c, on the other hand, does not make much sense to me, though if there is something I should be aware of (perhaps it makes more sense than I think it does?), I would also like to know.