Alpha helix is a coiled structure stabilized by:
a. covalent interactions within a polypeptide strands
b. hydrogen bonding within polypeptide strands
c. gravitational attraction of bulky r groups
d. hydrogen bonding between polypeptide strands
e. hydrophobic interactions between polypeptide strands

B.hydrogen bonding within polypeptide strands

Explanation:

Alpha helix is one of the secondary structures formed by polypeptide chains. It is formed when the backbone of a polypeptide is wound around a longitudinal axis. The side chains of the amino acids are oriented outwards. Alpha helix structure is stabilized by the formation of a maximum number of hydrogen bonds between the amino acids of the chain. The hydrogen atom linked to the electronegative nitrogen atom of a peptide bond form hydrogen bond with the electronegative carbonyl oxygen atom of the fourth amino acid.

In this way, every peptide bond except the ones present close to each end of the helix take part in hydrogen bonding and stabilize the structure.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.

d concave shape of wave rock

explanation:

the concave shape of wave rock is a very good evidence that supports the evolution of earth due to chemical weathering. if we talk about structure of   wave rock, studies have estimated that it is around 2.7 billion years old. it also serves as the foundation of evidence that it was the place where australian continent began to form.

how the structure of wave rock was created?

studies suggest that structure of wave rock was created when the surrounded area was started to weather due to which deposition of material started on the part of land which now contains wave rock.

the structure of wave rock is considered to be one of the best-known landforms in australia. at the height of around 15 cm, there can be found flared slopes which formulate the margins of hyden rock. it is now assumed by geologists that wave rock was created by subsurface water-induced weathering and not by glaciers, waves or wind.

due to deposition of material over the period of long time, the wave rock became tougher and tougher due to weathering from surrounding terrain. this makes it a physical evidence that supports   the evolution of earth due to chemical weathering.

hope it !