Which of these statements are qualitative? Which are quantitative? Explain your choice in each case.
(a) Sodium is a silvery-white metal.
(b) Aluminum melts at $660^{\circ} \mathrm{C}$.
(c) Carbon makes up about $23 \%$ of the human body by mass.
(d) Pure carbon occurs in different forms: graphite, diamond, and fullerenes.