It has been found that a particular "+ —" combination of frameshift mutations in gene x gives a wild-type phenotype, whereas a "— +" combination gives a mutant phenotype, even though the sites of insertion/deletion are the same. how might these results be explained?

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.

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