Detailed Instructions: In this task, you need to reverse all words of a given length in the sentence. The number of letters in a word determine its length. For example, the length of the word "apple" is 5.
Q: Sentence: 'a group of boats sitting in a lake by some trees'. Reverse all words of length '7' in the given sentence.
A:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
a group of boats gnittis in a lake by some trees