Write the formula used to calculate the standard deviation of a sample size more than thirty and less than thirty.

a) (σ = ∑(Xi - barX)²/n)
b) (σ = ∑(Xi - barX)²/n-1)
c) (σ = √∑(Xi - barX)²/n)
d) (σ = √∑(Xi - barX)²/n-1)