how many strings of four letters of the english alphabet contain exact
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How many strings of four letters of the English alphabet contain exactly two vowels? O P(5,4) \cdot\left(5^{2} \cdot 21^{2}\right) 4 \cdot\left(5^{2} \cdot 21^{2}\right) P(4,2) \cdot\left(5^{2} \cdot 21^{2}\right) O None of