Areaction produces a gas, which is collected in a glass cylinder. when a glowing splint is placed inside the container, it burns brightly, as shown. which gas was produced in the reaction?

If a burning splint is placed in a container and burns more brightly in the presence of some gas already in the cylinder, that gas would probably be Oxygen.  

Oxygen is the main component of air that we need to breathe. It is the most commonly available oxidizer. Besides combustion, it plays many roles in our life such as; the food we eat requires oxygen which helps in digestion and absorption of nutrients by the cells.  

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.