Which action characterized Earth's earliest atmosphere (atmosphere I) ? Helium and hydrogen gases escaped earth's gravity.
Increased oxygen allowed reptiles to thrive.
Heat energy was retained and water vapor condensed.
Ultraviolet light split water vapor into hydrogen and oxygen.

a and c are the answeres

brain development. a child's brain undergoes an amazing period of development from birth to three—producing more than a million neural connections each second. the development of the brain is influenced by many factors, including a child's relationships, experiences and environment.

explanation:

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.