1). To describe the length of a classroom, a student can use any unit
he feels like using. Some units will produce ridiculous numbers, though.
The most convenient numbers will result from using meters or centimeters.
If the room is, say, 20 feet long, then that's about 0.0061 kilometer, or
about 610 centimeters, or about 6.1 meters.
Meters is probably best.
2).
A). Kelvin: time. No. Kelvin is a unit of temperature.
B). second: time Yes. Second is a unit of time.
C). meter: volume No. Meter is a unit of length or distance.
D). gram: temperature. No. Gram is a unit of mass.
3). A plate near Hawaii grows about 11 cm per year.
So it grows 11 cm in the first year, and another 11 cm in the second year.
The total growth in 2 years is (11cm + 11cm) = 22 cm .
4). A slower plate in the Arctic moves slightly less than 2.5 cm per year.
It moves ...
slightly less than 2.5 cm in the 1st year
slightly less than 2.5 cm in the 2nd year
slightly less than 2.5 cm in the 3rd year
slightly less than 2.5 cm in the 4th year
slightly less than 2.5 cm in the 5th year
slightly less than 2.5 cm in the 6th year
slightly less than 2.5 cm in the 7th year
slightly less than 2.5 cm in the 8th year
slightly less than 2.5 cm in the 9th year
and
slightly less than 2.5 cm in the 10th year
for a grand total of
(slightly less than 2.5 cm x 10) = less than 25 cm in 10 years.