<em>The correct option is A) an increase in oxygen levels, which led to the extinction of many anaerobic organisms.</em>
Explanation:
Photosynthesis can be described as a process in which food (glucose) is produced by photosynthetic organisms by using carbon dioxide, water along with light energy from the sun. Oxygen is released as a by-product of this reaction. The process of photosynthesis initially allowed for oxygen gas to get accumulated in the air. This caused the rise of aerobic organisms who required oxygen for respiration. Before the photosynthesis organisms evolved, there was not enough oxygen gas on earth to support the life of aerobic organisms.
The correct answer is option C. Endocrine glands secrete hormones into the bloodstream because endocrine glands are ductless and the exocrine glands have ducts so they secrete their hormones into the ducts.
