for question b element C and D have the same atomic numbers.
Explanation:
element A and B are isobars because they have the same mass numbers but different atomic numbers. The different atomic numbers is as a result of the difference in proton number of each elements A and B. (A and B are two different elements).
element C and D are isotopes of the same element because they have the same atomic numbers (same number of protons) but differ in number of neutrons resulting in the different number of mass numbers. (C and D are the same elements)
The vapour pressure of a solution is lower than that of the pure solvent. And according to Raoult's law this depends of the amount of solute in the solvent. It is calculated according to the following equation
