What are the solutions for questions 17 through 21, including the equivalent resistance calculation?

For a converging lens, the focal length is positive. $P = \frac{1}{f}$

$$
P = \frac{1}{0.20 \text{ m}}
$$
$$
P = +5.0 \text{ D}
$$
The correct option is **B**.  
The final answer is $\boxed{+5.0 \text{ D}}$.

18. Given: Radius of curvature, $R = 2 \text{ m}$

$$
f = -\frac{2 \text{ m}}{2}
$$
$$
f = -1 \text{ m}
$$
The correct option is **C**.  
The final answer is $\boxed{-1 \text{ m}}$.

19. Ohm's law states that the voltage ($V$) across a conductor is directly proportional to the current ($I$) flowing through it, provided all physical conditions and temperature remain constant. The constant of proportionality is the resistance ($R$). The correct expression for Ohm's law is $V = IR$. The correct option is A. The final answer is $\boxed{V = IR}$.

20. The resistance ($R$) of a wire is given by the formula $R = \rho \frac{L}{A}$, where $\rho$ is the resistivity of the material, $L$ is the length of the wire, and $A$ is its cross-sectional area. From this formula, resistance is directly proportional to length. Therefore, an increase in length will increase the resistance of the wire. The correct option is D. The final answer is $\boxed{\text{increase in length}}$.

21. The SI unit of electric current is the ampere (A). The correct option is B. The final answer is $\boxed{\text{ampere}}$.

For the equivalent resistance calculation shown in the image:
The circuit shows three $2 \Omega$ resistors connected in parallel.

The calculation shown in the image for $R_{eq}$ is incorrect in its initial setup ($1 - \frac{1}{2} + \frac{1}{2} + \frac{1}{2} \Omega$), but the final numerical value of $0.67 \Omega$ is correct for three $2 \Omega$ resistors in parallel.