Data Sufficiency

if n is +ve which of the following is equal to
1/ sq root of n+1 - sq root of n

1.1
2.sq root 2n+1
3.sq rt n+1 / sq rt n
4.sq root of n+1 - sq root of n
5.sq root of n+1 + sq root of n

i understood the explanation but why is it not 1
i sq the equations the n cancels put 1 /1 ans is 1
I want to know what is wrong with tht methord