Which element is present in the highest amount in plants?

The major product in the following reaction is:

The acceleration of a moving body can be found from

Magnesium burns in an atmosphere of $CO_2$ because...

In the circuit shown in the figure,the reading of the voltmeter is $V_1$ when only $S_1$ is closed,the reading of the voltmeter is $V_2$ when only $S_2$ is closed,and the reading of the voltmeter is $V_3$ when both $S_1$ and $S_2$ are closed. Then:

Which of the following pairs is incorrect?