What Was the Temperature of the Universe When a Typical $\lambda$

Multiple Choice

What was the temperature of the universe when a typical CMB photon was no longer able to spontaneously create an electron and positron pair? Note that a photon has an energy equal to E = hc/ $\lambda$ , the CMB at any point in time has a blackbody spectrum that peaks at a wavelength $\lambda$ _peak = (2900 $\mu$ m K) /T, and the mass of a single electron is 9 * 10^-31 kg.

A) 2000 K
B) 400,000 K
C) 2 million K
D) 40 million K
E) 2 billion K