How did the appearance of microbial life on land increase the formation of a living soil during early earth, which in turn allowed for the evolution of land plants? a) microbes allowed plants to uptake oxygen-containing compounds through the process of oxygen fixation. b) microbes allowed plants to uptake carbon-containing compounds through the process of carbon fixation. c) microbes allowed plants to uptake hydrogen-containing compounds through the process of hydrogen fixation. d) microbes allowed plants to uptake nitrogen-containing compounds through the process of nitrogen fixation.

The correct answer is option D, that is, microbes allowed plants to uptake nitrogen-containing compounds via the process of nitrogen fixation.  

The majority of the conversion of nitrogen into ammonia, and further into proteins is attained by the microorganisms by the procedure known as nitrogen fixation.  

All the nitrogen-fixing organisms are bacteria (prokaryotes), some of them are free-living, that is, live independently of other organisms, like Azotobacter, and some live in symbiotic relationship with plants or with other organisms like Rhizobium.  

"do a testcross with a homozygous recessive plant."