The table shows the resistivity of some materials at 20°C. Which of the following is the best conductor of electricity?

Step 1: Translate Question 1. The question asks: "What is the focal length of a lens with a power of +2D?"

Step 2: Calculate the focal length. The power of a lens ($P$) is the reciprocal of its focal length ($f$) in meters. $P = \frac{1}{f}$ Given $P = +2 \, \text{D}$. $f = \frac{1}{P} = \frac{1}{2 \, D} = 0.5 \, \text{m}$

Step 3: Identify the correct option for Question 1. The calculated focal length is $0.5 \, \text{m}$. Comparing with the given options: (A) $0.5 \, \text{m}$ (B) $2 \, \text{m}$ (C) $0.2 \, \text{m}$ (D) $5 \, \text{m}$ The correct option is (A). The answer for question 1 is $\boxed{A) 0.5 \, m}$.

Step 4: Translate Question 2. The question asks: "Which of these materials is the best conductor of electricity?" The table provides the resistivity ($\rho$) of four materials (K, L, M, N) at $20^\circ \text{C}$.

Step 5: Analyze the resistivity values. A material with lower resistivity is a better conductor of electricity. We need to find the material with the smallest resistivity value. The given resistivity values are: • Material K: $\rho_{K} = 2.63 \times 10^{-8} \, \Omega \cdot \text{m}$ • Material L: $\rho_{L} = 5.20 \times 10^{-8} \, \Omega \cdot \text{m}$ • Material M: $\rho_{M} = 1.60 \times 10^{-8} \, \Omega \cdot \text{m}$ • Material N: $\rho_{N} = 6.84 \times 10^{-8} \, \Omega \cdot \text{m}$

Step 6: Compare the resistivity values. Comparing the numerical values ($2.63, 5.20, 1.60, 6.84$) while keeping the exponent $10^{-8}$ in mind, the smallest value is $1.60$. Therefore, Material M has the lowest resistivity.

Step 7: Identify the correct option for Question 2. The material with the lowest resistivity is M, making it the best conductor. Comparing with the given options: (A) N (B) K (C) M (D) L The correct option is (C). The answer for question 2 is $\boxed{C) M}$.