DNA is often compared to a twisted ladder. In this analogy, what forms the rungs of the DNA ladder?
A. Pairs of nitrogenous bases
B. Pairs of nucleotides
C. Pairs of sugars
D. Pairs of phosphate groups

The circles, or orbits, for electrons are called energy levels. each level can hold only a certain number of electrons. add electrons to each level until you can't add any more. how many electrons can each level hold?

Explain how scientists use geologic time to determine the age of landforms.

Which of the following types of stars is most likely to end up as a supernova? in graph a, the curve peaks at 800 nm, in the red section of the visible light spectrum. in graph b, the curve peaks at 550 nm, in the green section of the visible light spectrum. in graph c, the curve peaks at 450 nm, in the blue section of the visible light spectrum. in graph d, the curve peaks at 300 nm, in the violet section of the visible light spectrum. a b c d