What comes next in the following sequence?
B, C, E, G, __

Let's write down the numerical positions of each alphabet letter in the standard alphabetical order ($A=1,B=2,C=3,\dots$):
* $\text{B}=2$
* $\text{C}=3$
* $\text{E}=5$
* $\text{G}=7$

The sequence of positional numbers is $2,3,5,7,\dots$, which clearly represents consecutive prime numbers.
The next prime number after $7$ is $11$.

The $11^{\text{th}}$ letter in the alphabet is $\text{K}$.