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

[Q]: Sentence: 'a woman holds the reins of a saddled horse'. Find frequency of the letter 'm'
[A]: 1

[Q]: Sentence: 'a red fire hydrant opened and spilling water'. Find frequency of the letter 'd'
[A]: 4

[Q]: Sentence: 'a lone zebra is grazing in the dirt of his enclosure'. Find frequency of the letter 'n'
[A]:
4