The probability of having the combination of one head and one tail when flipping two coins is

1/2 or 50%

Explanation:

Odyssey ware answer of 1/4th was INcorrect. Answer 1/2 at 50% chance was correct on second graded attempt.

sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.