Scientists have hypothesized that oxygen began to build up in the earth's atmosphere after the development of living things with the ability to
a. breathe air.
b. form tissues.
c. photosynthesize.
d. reproduce sexually.

c. photosynthesize.

Explanation:

It is believed that it was life itself that originated all this oxygen present on the planet and promoted the change in the composition of the earth's atmosphere. This change would have begun, according to paleontologists, when the ability to perform photosynthesis emerged during the evolution of living beings. Commonly associated with plants, the ability to produce oxygen is also common to bacteria such as blue algae, unicellular beings that live in the oceans and, despite their name, are more similar to bacteria than algae. The great virtue of photosynthesis is to convert sunlight and carbon dioxide into energy for metabolism. As a byproduct, oxygen is released.

a. a primary source combines several different accounts of an event. b. a primary source generally benefits from having been written after an event.

explanation:

the tides are influenced by both the moon and the sun, it's easy to see that when the sun lines up with the moon and the earth, as during a new moon or full moon, the tidal effect is increased. these are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal. on the other hand, if the sun and the moon are 90 degrees apart in relation to an observer on earth as during the first quarter moon or third quarter moon, then high tides are not as high as they normally would be. this is because despite its greater distance, the sun's mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the moon's pull. this phenomenon of lower high tides is called a neap tide. if you need more information on this topic visit moonconnection.com.