Answer is: (B) \(y>x\)
\(x=\sqrt{21}-\sqrt{20}=\frac{21-20}{\sqrt{21}+\sqrt{20}}=\frac{1}{\sqrt{21}+\sqrt{20}}\)
\(y=\sqrt{18}-\sqrt{17}=\frac{18-17}{\sqrt{18}+\sqrt{17}}=\frac{1}{\sqrt{18}+\sqrt{17}}\)
\(\because \sqrt{18}+\sqrt{17}<\sqrt{21}+\sqrt{20}\)
\(\Rightarrow \frac{1}{\sqrt{18}+\sqrt{17}}>\frac{1}{\sqrt{21}+\sqrt{20}}\)
\(\Rightarrow y>x\)
