$1.$ Identify valid C# variable names and variable types from the given list and write a brief description of these types.

a$)$ Stock$\_$value

b$)$ f$($x$)$

c$)$ my grade

d$)$ string

e$)\_200$

f$)$ int

g$)$ abc@yahoo

h$)$ Too

i$)-3$x

j$)$ R&D

k$)$ double

l$)$ Low$\_$cost

m$)2013$

n$)$OK$$