Why did most Texan soldiers never cross the Mississippi River during the Civil War?

a.
Because they had no way of getting out of Texas once the railroad construction stopped.
b.
Because the Union had control of the Mississippi and would not let them pass.
c.
Because they were afraid of being out numbered by Union soldiers.
d.
Because it was important for them to stay and defend the ports along the Texas coast so that supplies could come in for the Confederacy.

PLEASE I NEED HELP !