Which of these characteristics is common only at the embryonic stage of fish and humans, and is suggestive of a common ancestor between the two? a) presence of lungs b) presence of a tail c) presence of gill slits eliminate d) presence of a vertebral column

The answer is; C

Most often some ancestral characters are preserved in an organism's development hence can be used to study evolutionary relationships. One of these features is the gill slits in embryos. These have been used to postulate that all land organisms, that exhibit gill slits in their embryonic development, emanated from a common fish ancestor. In humans, these gill slits later develop in the pharyngeal.

image result for what happens whenever a net force acts on an object

if an object has a net force acting on it, it will accelerate. the object will speed up, slow down or change direction. an unbalanced force (net force) acting on an object changes its speed and/or direction of motion. an unbalanced force is an unopposed force that causes a change in motion.