A fair coin is tossed three times. The random variable $x$ is defined to be $2^h,$ where $h$ is the number of flips that come up heads. For example, $x(HHT)=2^2.$ What is $E[x]?$

a$.1$

b$.\frac{3}{2}$

c$.2$

d$.\frac{27}{8}$