In Looking for Exoplanets, One Important Discovery Target Is Oxygen

In looking for exoplanets, one important discovery target is oxygen because:

A) oxygen was an important component of the original solar nebula, and the solar system could not have formed without it.
B) oxygen is an important part of water, one of the most important ingredients in Earth-based life.
C) oxygen's spectrum is part of the infrared spectrum where planets are brighter and stars are dimmer.
D) Earth's atmospheric oxygen was produced by living organisms, and we are unaware of any nonbiological method for producing an oxygen atmosphere.