Acceptability of 〜ようとしよう vs. 〜ことにしよう

Question

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:

a. 行【い】こうとしようよ

b. 行【い】くことにしようよ

c. 行【い】くようになろうよ

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.

Answer 1

Even without any context to go with, only [b. 行くことにしようよ] is correct as a phrase. We would never say [a. 行こうとしようよ] or [c. 行くようになろうよ] in any situation.

The problem with [a. 行こうとしようよ] is that it is double-volitional (行こう & しよう) and it is ungrammatical. It is grammatical to say 「行くとしよう」 or 「行こうとする」 in single-volitional, but not 「行こうとしよう」 in double. Though it is ungrammatical, it sounds a little "better" than [c. 行くようになろうよ].

As far as I understand, a. would translate to something like "let's try to go next week"

Your translation is single-volitional, not double. 「行こうとしようよ」 means something like "Let's try to try going next week.", which is why it is an incorrect answer.

[c. 行くようになろうよ] sounds pretty -- for lack of a better word -- "hilarious"; it is ungrammatical and it makes close to no sense. The 「なろうよ」 part is just out of place. One could say 「ともだちになろうよ！」= "Let's become friends!", but not 「行くことになろうよ」.