Which of the following is a form of active transmembrane transport?

• A. Osmosis of water through aquaporins driven by an osmotic gradient
• B. Facilitated diffusion of glucose through a GLUT transporter down its concentration gradient
• C. Simple diffusion of $O_2$ across the lipid bilayer down its concentration gradient
• D. ${\mathrm{Na}}_{+}$/$K_{+}$ ATPase pumping ions against their concentration gradients

Answer: (D)