<s>[INST] context: CO2 & Ammonia CPS-I causes condensation of CO2 and ammonia.Biosynthesis of urea occurs in five steps :1) Carbamoyl phosphate synthase-I (CPS-I), a mitochondrial enzyme, catalyzes the formation of carbamoyl phosphate by condensation of CO2 and ammonia. Two molecules of ATP are required for the reaction. CPS-I is the rate limiting enzyme of urea cycle. It is an allosteric enzyme and allosterically activated by N-acetyl glutamate.2) Ornithine transcarbamoylase catalyzes the formation of citrulline from carbamoyl phosphate and ornithine.3) Arginosuccinate synthase catalyzes the formation of arginosuccinate from citrulline and aspaate. This reaction requires 1ATP, but 2 high energy phosphate bonds are consumed as ATP is conveed to AMP + PPi. The amino group of aspaate provides one of the two nitrogen atoms that appear in urea (The other one is provided by ammonia NH4).4) Arginosuccinate lyase (arginosuccinase) catalyses the cleavage of arginosuccinate into arginine and fumarate. Fumarate enters in TCA cycle.5) Arginase catalyses the formation of urea from arginine by hydrolytic cleavage of arginine to yield urea and ornithine. Ornithine is thus regenerated and can enter mitochondria to initiate another round of the urea cycle. [/INST] question: Carbamoyl phosphate synthase - 1 acts on ? | options: A: Ornithine B: Carbamoyl phosphate C: CO2 & ammonia D: Arginine | answer: C </s>