If the Sun is assumed to be a uniform spherical plasma consisting of nucleons, with radius 7 × 10⁵ km and total mass 2 × 10³⁰ kg, calculate the mean free path λ = 1/nσ of solar neutrinos from the dominant reaction (2.35). Here n is the number of nucleons per unit volume and σ, the neutrino-nucleon cross-section, may be written σ = 0.7E_L × 10⁻⁴² m², where E_L is the neutrino laboratory energy in GeV.