Some agar was coloured pink using a pH indicator and used to make four agar blocks with different dimensions. The four agar blocks were placed in a beaker and covered with

1.0 mol dm^{- 3}

hydrochloric acid at a temperature of

20^{\circ} C

. The hydrochloric acid diffused into the agar blocks causing the blocks to become colourless. Which diagram correctly identifies the agar block that became colourless in the longest amount of time?

Block	Dimensions (not to scale)
A	10 mm $\times$ 10 mm $\times$ 10 mm
B	20 mm $\times$ 20 mm $\times$ 20 mm
C	40 mm $\times$ 20 mm $\times$ 10 mm
D	40 mm $\times$ 40 mm $\times$ 5 mm