During normal breathing, what is the usual stimulus for a person to take a breath? a. a decrease in pH of the cerebrospinal fluid

b. stimulation by norepinephrine

c. PCO2 in the blood drops below 40 mmHg

d. PO2 in the blood drops below 100 mmHg

sex cells are usually half's then they find another half and make a fertilize egg

maybe because its impossible for balloons to lift a whole entire house.

evidence and logical support is essential for a scientist to do in order to maintain credibility. this is because no one can believe without any logic and without any evidence.hence a scientist can maintain credibility by proving himself and by answering logically so experiments prove very beneficial. logical hypothesis give conclusion which is acceptable. therefore, there are two main factors to maintain credibility by a scientist is evidence and logical support.

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.