8. A solenoid has length \( L = 1\mathrm{m} \) and inner diameter \( d = 3\mathrm{m} \), and it carries a current \( i = 5\mathrm{A} \). It consists of 4 close-packed layers, each with 250 turns along length \( L \). What is B at its center? (10%) [Note: \( \mu_0 = 4\pi \times 10^{-7}\mathrm{T - m / A} \)]