A simple circuit consists of a known resistance $$R_A = 2\ \text{M}\Omega$$ and an unknown resistance $$R_B$$, both in series with a battery of $$9\ \text{V}$$ and negligible internal resistance. When the voltmeter is connected across $$R_A$$, it measures $$3\ \text{V}$$, but when the same voltmeter is connected across $$R_B$$ it reads $$4.5\ \text{V}$$. The voltmeter measures $$9\ \text{V}$$ across the battery. Considering that the voltmeter has a finite resistance $$r$$, the correct option is