Question 1
which of these is a period?

mesozoic
ceonzoic
triassic
precambrian

question 2
which series of words describes lengths of geologic time from longest to shortest?

eon > era > period
era > period > eon
period > eon > era
era > eon > period

question 3
how do scientists define the segments of the geologic time scale?

the scientists study fossils and rock layers to find major changes.
the scientists use fixed amounts of time for each segment.
they do both of these.
they don't do either of these.