$2{\mkern 1mu} {\mkern 1mu}  \le {\mkern 1mu} {\mkern 1mu} r{\mkern 1mu} {\mkern 1mu}  \le {\mkern 1mu} {\mkern 1mu} n{\mkern 1mu} $ for ${\rm{\{ }}{r^n}{\rm{\} }}{\mkern 1mu}  + {\mkern 1mu} 2{\mkern 1mu} \left( {\begin{array}{*{20}{c}}
n\\
{r{\mkern 1mu}  - {\mkern 1mu} 1}
\end{array}} \right){\mkern 1mu}  + {\mkern 1mu} \left( {\begin{array}{*{20}{c}}
n\\
{r{\mkern 1mu}  - {\mkern 1mu} 2}
\end{array}} \right){\mkern 1mu}  = {\mkern 1mu} .....$

• A

  $\left( {\begin{array}{*{20}{c}}{n\, + \,1}\\{r\, - \,1}\end{array}} \right)$

• B

  $\,2\,\left( {\begin{array}{*{20}{c}}{n\, + \,1}\\{r\, + \,1}\end{array}} \right)\,$

• C

  $\left( {\begin{array}{*{20}{c}}{n\, + \,1}\\r\end{array}} \right)$

• D

  $\,\left( {\begin{array}{*{20}{c}}{n\, + \,2}\\r\end{array}} \right)$