According to Dobereiner's law of triads, the atomic mass of the middle element of a triad is approximately the arithmetic mean of the atomic masses of the other two elements. Given atomic masses Li = 7, Na = 23, K = 39; F = 19, Cl = 35.5, Br = 80; He = 4, Ne = 20, Ar = 40; N = 14, P = 31, As = 75, which of the following is a valid Dobereiner triad?