During which stage does the birth rate begin to decline?

During stage 3 - late expanding (of demogrpahic transition model)

Explanation:

During stage 3, birth rate begins to decline as infant mortality is lower and women have more access to education, family planning, and contraceptives.  Children are not needed as "free labor" as they might have been in earlier stages.

absorption lines are where light has been absorbed by the atom thus you see a dip in the spectrum whereas.

emission spectra have spikes in the spectra due to atoms releasing photons at those wavelengths.

explanation:

kepler used the data from tycho blahe (his mentor) to construct the laws of planetary motion.