  We examine the electronic properties of 2D electron gas in black phosphorus
multilayers in the presence of a perpendicular magnetic field, highlighting the
role of in-plane anisotropy on various experimental quantities such as ac
magneto-conductivity, screening, and magneto-plasmons. We find that resonant
structures in the ac conductivity exhibits a red-shift with increasing doping
due to inter-band coupling, $\gamma$. This arises from an extra correction term
in the Landau energy spectrum proportional to $n^2\gamma^2$ ($n$ is Landau
index), up to second order in $\gamma$. We found also that Coulomb interaction
leads to highly anisotropic magneto-excitons.
