Urea is produced from a mixture of—

• a$\mathrm{N_2}$ and $\mathrm{H_2}$
• b$\mathrm{CO_2}$ and $\mathrm{NH_3}$
• c$\mathrm{CO}$ and $\mathrm{NH_3}$
• d$\mathrm{N_2}$ and $\mathrm{O_2}$