In this task, you need to count the number of times the given letter appears in the given sentence.

[Q]: Sentence: 'a group of tennis players on a tennis court'. Find frequency of the letter 'o'
[A]: 4

[Q]: Sentence: 'a yellow and green train on a train track in a wooded area'. Find frequency of the letter 'o'
[A]: 4

[Q]: Sentence: 'four people sitting on a ski lift going up a snowy mountain'. Find frequency of the letter 'p'
[A]:
3