The letter 's' is one of the most common consonants in English. It frequently appears at the end of words, and could potentially be the last letter in this 7-letter word. Additionally, there are many common words with 's' following 'a' in patterns like "as" or "ast". Given that we've already tried other common vowels (e, i) and a few consonants (r, t), 's' is a logical next choice to maximize our chances of finding another letter in the word.