The component used to vary resistance in an electric circuit is called: 29. The total resistance of the circuit in Figure 4 is

28. The component used to vary resistance in an electric circuit is called:
A component designed to vary resistance in an electric circuit is known as a rheostat.

$$\boxed{\text{C. Rheostat}}$$

29. The total resistance of the circuit in Figure 4 is

Step 1: Calculate the equivalent resistance of the two parallel resistors ($R_p$).
The resistors are $3 \text{ Ω}$ and $6 \text{ Ω}$.

$$\frac{1}{R_p} = \frac{1}{3 \text{ Ω}} + \frac{1}{6 \text{ Ω}}$$ $$$$