When doing medical research with human subjects, which four limitations are unavoidable? A- It’s often impossible to repeat trials on the same subjects.
B- Subjects may report an inaccurate medical history.
C- It can be difficult to control all possible variables.
D- It’s impossible to come up with testable scientific questions for human subjects.
E- There are ethical or privacy concerns to consider.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.