The root mean square speed of smoke particles each of mass 5×10⁻¹⁷kg in their Brownian motion in air at N.T.P is?
A. 3×10⁻² m s⁻¹
B. 1.5×10⁻² m s⁻¹
C. 3×10⁻³ m s⁻¹
D. 1.5×10⁻³ m s⁻¹