According to the textbook's discussion of anxiety:
a) panic attacks are called the fear of fears because people feel that they are dying.
b) in many situations it is sensible to feel anxiety, and in fact, these emotions are adaptive.
c) post-traumatic stress disorder occurs within the first two days after a trauma.
d) it is unusual for a mentally healthy person to experience a panic attack.

A) panic attacks are called the fear of fears because people feel that they are dying.

Explanation:

Anxiety is a situation of fear and tension which can be felt mentally as well as physically. Discussion of anxiety can be define as the panic attacks which are associated with the fear of dying.

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.