Question

1. which is not a characteristic of all vertebrates?

paired appendages
an exoskeleton
a backbone
a well - developed brain protected by a skull

Explanation:

Vertebrates are defined by having a backbone (vertebral column), and all have a well-developed brain protected by a skull. While most have paired appendages, some primitive vertebrates (like hagfish) lack them, but the key non-universal trait here is an exoskeleton: vertebrates have an internal endoskeleton, not an external exoskeleton (which is characteristic of arthropods).

Answer:

B. an exoskeleton