If $A,$ $B,$ and $C$ are $2 \times 2,$ $2 \times 6,$ and $6 \times 7$ matrices respectively, determine which of the following products are defined. For those defined, enter the size of the resulting matrix (e.g. "3 x 4", with spaces between numbers and "x"). For those undefined, enter "undefined".

$CB$:

$AC$:

$A^2$:

$AB$: