This simplified H-R diagram has luminosity increasing on the vertical axis and temperature increasing to the left on the horizontal axis. The Main sequence is indicated by the grey shaded line from upper-left to lower-right. There are 4 stars shown. What is happening at A? Star A is in the lower-left. Star B is in the upper-left, within the grey area. Star C is in the lower-right, within the grey area. Star D is in the upper-right. Group of answer choices Not fusing anything, but is just a hot ball of Carbon Fusing hydrogen in a shell around a helium core Fusing hydrogen in its core Fusing helium in its core and hydrogen in a shell