dse 2010 doubt

[shr123]

suppose x1 and x2 are real valued variables with f as their common density function. suppose (x1 x2) is s sample generated by these random variables the expectation of the no of observation in the sample will fall within a specified interval [a b] is
(integral of f(x) over a and b)^2
x^2f(x)dx over a b
2(f(x)dx over a and b)
xf(x)dx over a b

[Sris]

i think its a

[shr123]

the final answer accoding to the answer key is c

[L]

check in this pdf.
notes_1.pdf

[shr123]

thanks