Which of the Following Has a Much Lower Concentration in the Cerebrospinal

Which of the following has a much lower concentration in the cerebrospinal fluid (CSF) than in cerebral capillary blood?

A) $\mathrm{Na}^{+}$
B) $\mathrm{K}^{+}$
C) Osmolarity
D) Protein
E) $\mathrm{Mg}^{2+}$