The vector 'letters' is a built$-$in vector to $\text{'}$R$\text{'}$ and contains the lower case English alphabet.

a$.$ Extract the $9$ th element of the 'letters' vector.

b$.$ Extract the sub$-$vector that contains the $9$ th$,11$ th$,$ and $19$ th elements.

c$.$ Extract the sub$-$vector that contains everything except the last two elements.

:$\text{'}\{r\}$

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