The expression $1+\sqrt[\phantom{2}]{2}+\sqrt[2]{3}is$ equivalent $toA.{\displaystyle \underset{n=1}{\overset{3}{}}}\sqrt[\phantom{2}]{n}B.{\displaystyle \underset{n=1}{\overset{3}{}}}{n}^{n}C.{\displaystyle \underset{n=1}{\overset{3}{}}}{n}^{-n}D.{\displaystyle \underset{n=1}{\overset{3}{}}}{n}^{\frac{1}{n}}$