Why is a DNA molecule called a double helix?
A. Each nucleotide has two main parts: a base and a phosphate.
B. The sugar and phosphate parts are the same in all DNA molecules.
C. There are two kinds of nitrogen bases in a DNA molecule.
D. A DNA molecule always has two strands of nucleotides.

not only is it colder, it also becomes darker, less salty, and more dense.

answer: b

coz red blood cells marrow and circulates the body for 120 days.

answer: the correct option is d. enzyme

lactase and carbonic anhydrase are the examples of enzymes.

lactase is an enzyme, which is present in the small intestine of mammals. it in the digestion of the disaccharide, lactose ( milk sugar) by converting it into two monosachharide, which are glucose and galactose.

carbonic anhydrase is present in the red blood cells. this enzyme plays significant role in the transport of co₂. it in the conversion of co₂ and h₂o into carbonic acid, which is then dissociated into bicarbonate ions and hydrogen ions.