In this English lesson, I explain how three prepositions -- at, on, and in -- are used in relation to time. A lot of students get confused with these words. If you have any questions about this lesson, leave a comment below.
The skin is the interface between the organism and the external environment, acting as its first barrier. Thus, this organ is constantly challenged by physical stimuli such as UV and infrared radiation, visible light, and temperature as well as chemicals and pathogens. To counteract the deleterious...