An atomic mass unit is equal to
a.
one-half the mass of a hydrogen atom.
b.
one-fourth the mass of a lithium atom.
c.
one-twelfth the mass of a carbon-12 atom.
d.
one-fifteenth the mass of a nitrogen-15 atom.

c.  one-twelfth the mass of a carbon-12 atom.

Explanation: An atomic mass unit ( amu) is defined as of the mass of an atom of carbon with mass number 12 , which is the most abundant isotope of carbon. Thus the carbon-12 atom has six protons and six neutrons in its nucleus.

1 amu is approximately equal to or . The mass of an atom in amu is roughly equal to the sum of the number of protons and neutrons in the nucleus.

amu is used to differentiate between isotopes as they would have different mass.

unbalanced force

explanation:

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.