问题详情

The primary purpose of the passage is to explain______.
A．the background and career of the astronomer Henrietta Leavitt
B．how and why various categories of stars vary in brightness
C．important uses of the camera as an astronomical tool
D．how a particular method of measuring astronomical distances was created此题为多项选择题。请帮忙给出正确答案和分析，谢谢！