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

The question asks: "What is the focal length of a lens with a power of +2D?"

The power of a lens ($P$) is the reciprocal of its focal length ($f$) in meters.

Given $P = +2 \, \text{D}$.

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 $\boxed{\text{A) } 0.5 \, \text{m}}$.

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 $20^\circ \text{C}$.

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: $\rho_{\text{K}} = 2.63 \times 10^{-8} \, \Omega \cdot \text{m}$
• Material L: $\rho_{\text{L}} = 5.20 \times 10^{-8} \, \Omega \cdot \text{m}$
• Material M: $\rho_{\text{M}} = 1.60 \times 10^{-8} \, \Omega \cdot \text{m}$
• Material N: $\rho_{\text{N}} = 6.84 \times 10^{-8} \, \Omega \cdot \text{m}$

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.

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 $\boxed{\text{C) M}}$.