What structure acts like the our skin monitoring what goes in and out of every single cell? Help me please

Select the situation bellow that would produce a total displacement of zero. a. a trip to the moon and then to mars b. a horse galloping from one end of a field to another c. the criss-crossing path of a bug as it flies from flower to flower d.a round-trip ride to school and back

Which represents a negative impact of technology

Which chemical equation is unbalanced? which chemical equation is unbalanced