The nine different classes of vertebrates include
a. four fish classes and amphibians, reptiles, birds, whales and mammals.
b. four fish classes and amphibians, reptiles, birds, mammals and primates.
c. five fish classes and amphibians, reptiles, birds, and mammals.
d. five fish classes and amphibians, reptiles, monkeys, and mammals.

Covalent bond is the correct answer

Iguess i would say diagrams would be most difficult subject too correct

It depends on the surface of the levels. the rougher the surface, the greater is the frictional force. for rougher surfaces, you would need to exert greater force to move the object. the concept is that, you must overcome the frictional force in order to bring the object into motion. this is the reason why we add lubricants to the joints so they could easily move. this answer was on