物理:電路 把開關S斷開?

Because R2 and R3 are in parallel, the equivalent resistance is smaller than R3. Hence, after S is opened, only R3 remains in the circuit. The total resistance of the circuit increases. Thus the current decreases.

Following the decrease of the main current, the voltage across R1 drops. Because the potential of point B is fixed at earth potential (0 volt), the potential at A thus decreases. Statement (1) is correct.

With R2 removed from the circuit (due to opening of switch S), R3 takes all the current that flows in the circuit (notice that when S is closed, R2 takes most of the current because R2 < R3). Statement (2) is wrong.

With the increase of current flows through R3, the the voltage across R3 increases. Because the potential at the left hand side of R3 is fixed at earth potential, an increase of voltage across R3 leads to the potential at C lower than before. Statement (3) is correct.
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The potential at B is earth potential, why not? The diagram clearly shows that point B connects to earth.