Answer the following questions regarding mass transport:

$($a$)$ For a binary solution, if the molar average velocity is $0,$ what is the relationship between $N_A$ and $N_B?,(3\%)$

$($b$)$ For a binary system, the diffusion flux $N_A$ can be described as the contributions of two terms. Write down the two terms and state their physical meanings.

$(4\%)$

c$)$ What are the molecular diffusivities of the three transport phenomena?

$(6\%)$

d$)$ Describe the assumptions of the two$-$resistance theory.

$(5\%)$