A $10\,\text{cm}^3$ portion of $0.30\,\text{mol dm}^{-3}$ $\text{Tl}^+\text{NO}_3^-$ needed $20\,\text{cm}^3$ of $0.10\,\text{mol dm}^{-3}$ acidified $\text{NH}_4\text{VO}_3$ in order to oxidise it to $\text{Tl}^{3+}$ in solution. Vanadium is the only element that is reduced in this reaction. What is the oxidation number of the vanadium in the reduced form?
- A$+1$
- B$+2$
- C$+3$
- D$+4$