DNA is double helix and `:`

Each of the following questions/statements has four suggested answers. Rewrite the correct answer in each case: DNA is a double helix and

DNA in eukaryotes is a double helix and:

In a DNA double helix :

DNA double helix unwinded by enzyme

One turn of B-DNA double helix spans a distance of

the fact that a purine base always pairs through hydrogen bonds with a pyrimidine base in the DNA double helix leads to