All animals have a system for exchanging carbon dioxide and oxygen with their surroundings. What do all of these systems have in common?

A. All involve the active transport of gases across membranes.
B. All include airways, such as the trachea, that carry air to the mouth.
C. All involve gas diffusion across moist, selectively permeable membranes.
D. All involve organs or tissues that are dedicated to gas exchange and fulfill no other functions.

A.All involve the active transport of gases across membrane

measuring light intensity

light meters can be used to measure light intensity. the meter is held at the soil surface and pointed in the direction of the maximum light intensity, and then the meter is read.

errors can be made when measuring light intensity by accidentally shading the light meter. the reliability of the results can be checked by taking many samples.

answer: 20 percent

the fish culture has become a developing commercial practice in developing countries which is already prominent in developed countries of the world. the major fish protein demand is being supplemented in developing countries is   fishing to meet the needs of the growing population. therefore, in some developing nations, the percentage of animal protein from fish in the diet can be as high as 20 percent.