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The sentence 微笑み選ぼう is part of the lyrics of a song called
[錆びついたマシンガンで今を撃ち抜こう](http://j-lyric.net/artist/a0039c4/l007405.html).

I'm wondering why it is correct in grammar because it seems to me that there
is supposed to be a を between 微笑み and 選ぼう.

== Update ==  
Here is the shorter version of the song for interpreting the meaning of this
sentence: [link](https://www.youtube.com/watch?v=HX9evhBKxf4).
CC BY-SA 3.0
2016-05-20T14:27:19.393
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question
particle-を song-lyrics
Why is 「微笑み選ぼう」 correct in grammar?
No for 微笑みを選ぼう。 In this case, '微笑み' is not object for choice.

微笑み選ぼう is more precisely 微笑み、選ぼう。

It means, With smile, choose (trip to uncharted town)!

It is hard to interpret this by grammatical analysis. Only semantic analysis
can get object of choice. I think.
CC BY-SA 3.0
2016-05-20T14:50:59.600
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It is omitted を. We often omit a post-positional particle especially in spoken
language.

For example, 本(を)買いに行く, 昨日(は),学校に行った, 明日どこ(へ)行く?, 何(を)食べようか?, etc.
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answer