# For the following Assertion and Reason, the correct option is

Question:

For the following Assertion and Reason, the correct option is

Assertion (A): When $\mathrm{Cu}$ (II) and sulphide ions are mixed, they react together extremely quickly to give a solid.

Reason (R): The equilibrium constant of $\mathrm{Cu}^{2+}(\mathrm{aq})+$ $\mathrm{S}^{2-}(\mathrm{aq}) \rightleftharpoons \mathrm{CuS}(\mathrm{s})$ is high because the solubility product is low.

1. (A) is false and (R) is true

2. Both $(\mathbf{A})$ and $(\mathbf{R})$ are false

3. Both (A) and (R) are true but (R) is not the explanation for (A)

4. Both (A) and (R) are true and (R) is the explanation for (A)

Correct Option: , 4

Solution:

Cesium has lowest ionisation enthalpy and hence it shows photoelectric effect to the maximum extent. So, it is used in photo electric cell.