In the Previous Two Problems, the Values $u _ { i , 1 }$

In the previous two problems, the values $u _ { i , 1 }$ depend on the values $u _ { i , - 1 }$ . How do you calculate those values?

A) Use a forward difference approximation in $t$ along the line $t = 0$ .
B) Use a backward difference approximation in $t$ along the line $t = 0$ .
C) Use a central difference approximation in $$t$$ along the line $t = 0$ .
D) Use a forward difference approximation in $x$ along the line $t = 0$ .
E) Use a backward difference approximation in $x$ along the line $t = 0$ .