*Q:20 Correct Answer* ✅ *(1) Both A and R are true and R is the correct explanation of A* > *Explanation:* All vertebrates are chordates because they have a notochord at least during the embryonic stage. However, not all chordates are vertebrates — some, like urochordates and cephalochordates, do not develop a vertebral column. > In vertebrates, the notochord is present during the embryonic stage but is later replaced by a cartilaginous or bony vertebral column, which is a defining feature of the subphylum Vertebrata.