Why does the twilight zone of the ocean have the greatest variation in temperature?

This is because twilight or mesopelagic zone (extending from 200 meters to 1,000 meters) is the zone that contains the thermocline.

A thermocline or the thermal layer is a layer in an ocean or lake where the temperature changes more rapidly with depth than in the other layers. The thermocline in twilight zone divides the upper mixed layer from the calm deep water below.

hormones in blood stream

image: dtc.ucsf.edu

endocrine glands secrete hormones into the blood stream near them; the hormones then travel in our bloodstream until it reaches its destination, called a target cell, in distance parts of the body. in the target tissue, hormones lock onto the target’s plasma membrane, called the receptor site.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.