Two lengths, $a$ and $b$, are measured to be $51 \pm 1{\text{ cm}}$ and $49 \pm 1{\text{ cm}}$ respectively. In which of the following quantities is the percentage uncertainty the largest?

A.     $a + b$

B.     $a - b$

C.     $a \times b$

D.     $\frac{a}{b}$

B