Which of the following is an example of active transport across a cell membrane?

• A. Transport of ${\mathrm{Na}}_{+}$ out of the cell and $K_{+}$ into the cell by the ${\mathrm{Na}}_{+}$/$K_{+}$ ATPase
• B. Facilitated diffusion of glucose through a GLUT transporter down its concentration gradient
• C. Movement of $O_2$ across the lipid bilayer down its concentration gradient
• D. Osmosis of water through aquaporins toward higher solute concentration

Answer: (A)