Helium gas in an ideal Otto cycle is compressed from 20°C and 2.5 to 0.25 L, and its temperature increases by an additional 700°C during the heat addition process. The temperature of helium before the expansion process is
(a) 1790°C
(b) 2060°C
(c) 1240°C
(d) 620°C
(e) 820°C