How are astronomers able to explore the layers of the Sun below the photosphere?
a. Astronomers study strong spectral lines of the Sun in the UV and the X-ray regions.
b. Astronomers study vibrations of the surface of the Sun caused by movements deep in the Sun and propagated outward like sound waves.
c. Astronomers study solar wind produced by the Sun.
d. Astronomers study formation of granules and changes in the chromosphere and the corona.