Economics is tough, can I get some help please?

[Nader G]

1) A natural monopoly is defined as an industry in which one firm..?

a. can produce the entire industry output at a lower average cost than a larger number of firms could.
b. can produce the entire industry output at a lower marginal cost than a larger number of firms could.
c. is very large relative to other firms that could enter the industry.
d. can earn higher profits if it is the only firm in the industry rather than if other firms also enter the industry.

2) At a given output level, a monopolist earns a profit only if the...?
a. slope of its TR curve exceeds the slope of his TC curve.
b. height of its MR curve exceeds the height of his MC curve.
c. height of its demand curve exceeds the height of his MR curve.
d. height of its demand curve exceeds the height of his ATC curve.

THANKS