The sum of 0.01 + 0.81 + 1.21 + 0.0009 = ?

Answer: Option B

$$\begin{array}{rl}& \text{According to the question,}\\ & \sqrt{0.01}+\sqrt{0.81}+\sqrt{1.21}+\sqrt{0.0009}\\ & =0.1+0.9+1.1+0.03\\ & =2.13\end{array}$$