We know that the atmosphere became more oxygen-rich around 1.8 billion years ago (ga) because of geologic evidence such as choose one:

a. redbeds that formed after 1.8 ga.

b. abundant fossils of land plants, signifying the start of photosynthesis.

c. bifs that occurred prior to 1.8 ga but disappeared from the rock record after that time.

d. the presence of abundant pyrite after 1.8 ga.

A. redbeds that formed after 1.8 Ga.

Explanation:

Red beds are known to be as the rocks which are sedimentary which are red in colour and contain high amount of ferric oxides.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.

tsd is temerature dependent sex determination.

tsd: it is a type of environmental sex determination in which the temperature experienced during embryonic/ larval development (ambient temperature for development) determine the sex of the offspring.

crocodile displays the tsd where the ambient temperature of the developing egg determines the individual's sex. for example in the american alligator's eggs, incubation at 33 ºc produces mostly males, while incubation at 30 ºc produces mostly females. so if the american alligator's egg will get the ambient temperature (33 degree celcius)during embryonic development from egg, the offspring will be male and same for the female sex determination.