All of the oxygen that we breathe is produced by plants.
a. true
b. false

The given statement is true.

Explanation:

The oxygen that is present in the atmosphere has two main sources:

Terrestrial Plants: As we know that the plants make use of sunlight, water, carbon dioxide and chlorophyll in a process termed as photosynthesis and make glucose and releasing oxygen into the atmosphere which is then used by animals for respiration. This is one of the major source of oxygen in the atmosphere.

Oxygen released from Oceans: In the oceans of the earth, water bodies ,seas tiny plants termed as Phytoplankton are present that are present near the surface of the water bodied that also release oxygen by photosynthesis and account for about 50% of the world's oxygen supply.