Levene made which contribution to our understanding of dna structure? a. he determined that the nucleus contains dna. b. he determined that dna contains four nitrogenous bases. c. he determined that dna consists of nucleotides. d. he determined that the nitrogenous bases of dna are present in regular ratios.

The correct answer is  b. He determined that DNA contains four nitrogenous bases.

Explanation:

Levene was an American biochemist who studied the structure of DNA in the early 1900s and in 1920 he found that DNA consists of four nitrogenous bases that are Adenine, Guanine, Thymine, Cytosine.

He also found deoxyribose sugar and phosphate group and said that nucleotide is the basic unit of DNA which contains a ribose sugar with attached nitrogenous base and phosphate group.

He also concludes that an equal amount of adenine, guanine, thymine, and cytosine makes the DNA which is now known as tetranucleotide hypothesis. So the correct answer is b. He determined that DNA contains four nitrogenous bases.  

carbon

explanation:

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.