If X has distribution function F(t) =

⎧⎪⎨⎪⎩

0, t < 0, t∕2, 0 ≤ t < 2, 1, t ≥ 2, then the standard deviation of X is

(a)

√

1 3

(b)

√

2 3

(c) 1 (d)

√

3 2

(e)

√

3