B. One
because an independent variable is something you change. So in a scientific experiment you only change one thing. If you happen to change more than one thing then the test becomes unfair therefore you should repeat and make sure that only one thing is changed this time.
Answer:
Details about DNA are given in the explanation section. Hope it will be helpful for you.
Explanation:
DNA, or deoxyribonucleic acid, is the hereditary element in humans and almost all other organisms. Nearly every cell in a person’s body has the same type of DNA. Most DNA is found in the cell nucleus (nuclear DNA), but a small quantity of DNA can also be found in the mitochondria (mitochondrial DNA or mtDNA).
The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same type in all people.
DNA bases pair up with each other, A with T and C with G, to form units that are called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. A base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix.
A valuable feature of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases.
Food webs normally start at the bottom and work their way up. Like the plants will be at the bottom, and then each level shows the animals getting bigger while losing energy. You continually lose energy as you work your way up an ecosystem, so normally there are only three or four levels. I hope this helps!
