The energy of an electron in a band as a function of its wave vector k is given by \(\rm E(k)=E_0-B\)\(\rm \left(\cos k_x a+\cos k_y a+\cos k_z a\right)\), where E0, B and a are constants. The effective mass of the electron near the bottom of the band is
1
\(\frac{2 \hbar^2}{3 B a^2} \)
2
\(\frac{\hbar^2}{3 B a^2} \)
3
\(\frac{\hbar^2}{2 B a^2} \)
4
\(\frac{\hbar^2}{B a^2}\)