If two independent sufficiently large samples are taken from two populations with known population standard deviations, the sampling distribution of the difference, sample mean (/x)1 - sample mean...

If two independent sufficiently large samples are taken from two populations with known population standard deviations, the sampling distribution of the difference, sample mean (/x)1 - sample mean (/x)2 , between the two sample means /x1 and /x2

a. can be approximated by poisson distribution

b. will have a variance of one.

c. can be approximated by a normal distribution

d. will have a mean of one.

I would like to know why the answer is the answer and thank you.