201902180835 Electric circuit diagrams (Elementary) Q3

The blogger claims no originality of his question below.

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In the figure above, when the key is closed, which of the following descriptions about the circuit is true?

1. If $A$ is an ammeter and $B$ is a voltmeter, then resistors $R_1$ and $R_2$ are in parallel.
2. If $A$ is an ammeter and $B$ is a voltmeter, then resistors $R_1$ and $R_2$ are in series.
3. If $A$ is a voltmeter and $B$ is an ammeter, then resistors $R_1$ and $R_2$ are in parallel.
4. If $A$ is a voltmeter and $B$ is an ammeter, then resistors $R_1$ and $R_2$ are in series.

Answer. A