Which of the following pairs of instruments are null instruments?

1. Which of the following pairs of instruments are null instruments?
A null instrument measures a quantity by balancing it against a known quantity, indicating a zero (null) deflection when balanced. A potentiometer measures potential difference by balancing it against a known potential difference, using a centre-zero galvanometer to detect the null point (zero current). This pair works together as a null measurement system. While a Wheatstone bridge is also a null instrument, a spring balance is a direct-reading instrument, not a null instrument.

The correct option is C.

2. A lethal dose of plutonium-239 isotope from radioactive waste is of mass $2.00 \text{ mg}$ and has an activity of $4.59 \times 10^6 \text{