Unlike images produced by a convex spherical mirror, images produced by concave spherical mirrors
a. are always going to be displayed upright, not inverted.
b. can be real or virtual depending on the object's placement.
c. are located only by the mirror.
d. are always going to be virtual.

Answer;

B. can be real or virtual depending on the object's placement.

Explanation;Unlike images produced by a convex spherical mirror, images produced by concave spherical mirrors can be real or virtual depending on the object's placement.Concave mirror and convex mirror are two types of spherical mirrors, where a convex mirror has a reflecting surface that bulges outside while a concave mirror has a reflecting surface that bulges inwards.Convex mirror forms images that are virtual and diminished, while a concave mirror forms an image that may either be real or virtual, diminished or enlarged depending on the position of the object from the mirror.

Answer (d) the universe had cooled enough for atoms to combine, protons and neutrons were too far apart to form nuclei;

Amicroscope, because these cells are so small that you would not be able to see them, or what they attack without it.