Select all the sentences that are open sentences.

A. $x > 2$

B. $x/2 = c$

C. $x = a$

D. $x/2 = c \:$ iff $\: x = 2c$

E. $2 + 1 = 3$

F. $x-a = b \:$ iff $\: x = b+a$

G. None of the above