A

As spoken by the Fourth Doctor, A was the first letter of the alphabet, preceeding B. (TV: The Masque of Mandragora)

In November 1963, science teacher Ian Chesterton set his Coal Hill School class a problem with the letters A, B and C representing the three dimensions. However, Susan Foreman pointed out that he had not acknowledged two additional dimensions: D, which Ian reasoned was time, and E, which Susan revealed was space. (TV: An Unearthly Child)