Answer:
Option C. is correct.
Explanation:
According to Chargaff's rules, DNA (Deoxyribonucleic Acid) from any species of an organism should have purine and pyrimidine bases in the ratio 
This ratio is very important for solving the structure of DNA.
As per this rule, DNA is double-stranded, and covalent bonds between identical nucleotides hold the strands together.
So, Option C. is correct.
The answer is called photosynthesis. Hope this helps.
If CO2 was in very small amounts then it would be the limiting factor of photosynthesis, this means the process will take place at a much slower rate because it is lacking one of the raw materials it needs for the process to occur. To find the optimum light intensity you really need all other factors to be at optimum levels or in abundance.