Each question is a WordPress post so it is up to the sitemap you have created to link to this post and it will be googled. On the other hand even when you submit a sitemap to Google it does not necessarily index everything.
If you install XML-Sitemap which is a popular sitemap plugin you will notice that the answers are one of the option to select when building your sitemap
Allows users to post a question, comment or answer (Q&A) in your discussion forum in Stackoverflow style. Our Question and Answer discussion forum plugin also includes social media registration like Facebook, LinkedIn or Google+ login. This Q&A enhancement has many other unique features that allow administrators to customize the display of visitors' queries and comments and rank responses according popularity or number of votes. This product is favorably reviewed and available in the WordPress Repository.