$ cat AtoF
A
B
C
D
E
F

Print only odd lines

sed -n 'p;n' AtoF
A
C
E

-n suppress automatic printing of pattern space, p print the current pattern space, n read/append the next line of input into the pattern space.

Alternatively:

$ sed -n 1~2p AtoF
A
C
E

-n suppress automatic printing of pattern space, 1~2 from the 1st line match every line every 2 steps, p print the current pattern space. sed -n 0~p has the same effect . print only the even lines.

Print only even lines

$ sed -n 'n;p' AtoF 
B
D
F

or

$ sed -n 2~2p AtoF
B
D
F

0~2p would also work. I prefer