The true length of a rod is $2.80\ \text{m}$. Instrument A gives $2.50\ \text{m}$, $2.51\ \text{m}$ and $2.50\ \text{m}$. Instrument B gives $2.7\ \text{m}$, $2.9\ \text{m}$ and $2.8\ \text{m}$. Which statement is right?