A test-cross is used to determine the genotype of a phenotypically dominant individual. To do this you mate the unknown individual with a individual and examine the phenotype of its offspring.

Explanation:

1. Test cross is done To identify whether an organism exhibiting a dominant trait is either homozygous or heterozygous for a specific allele, a scientist can perform a test cross.  

2. The organism which we want to check is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined.

3. Performed with an unknown dominant phenotype and homozygous  recessive.

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.