Why is the structure of dna called a double helix? a.because each of the curved dna subunits appears twice whenever it appears b.because dna is made of two strands of nucleotides wound together in a spiral. c.because two copies of dna are inherited, one each from the mother and father d.because the number of copies of dna doubles with each round of pcr submit for grading save and finish later?

The structure of DNA is called a double helix because DNA is made up of two strands of nucleotides wound together in a spiral. 

DNA has two strands that run opposite of each other and twist together much like a twisted ladder. Each backbone of the ladder is made up of alternating groups of sugar and phosphate groups.  

C. That jellyfish have stinging cells, while comb jellies do not

Explanation:

Comb Jellies to the untrained eye are the same as Jellyfish but as the question says, they are different in that they belong to different phyla.

The Comb Kelly belongs to the Ctenophora Phylum while the Jellyfish belongs to the Coelenterate phylum.

Perhaps the largest difference between them is that Ctenophora do not have stinging cells as opposed to members of the Coelenterate Phylum that do.