I'd go with the third one because the products of both of them are oxygen and carbon dioxide.
<span>overall, around twenty million teenagers contract an STI (sexually transmitted infection) each year. </span>(The half of them are from U.S.)<span>
A sexually transmitted infection (STI) is an infection that is transmitted between partners during different forms of sexual intercourse. This infection can lead to an infectious disease, formerly called venereal disease (the name comes from Venus, goddess of love)All sexual practices that involve mutual genital or oro-genital contact with another person, or genital fluids, are considered to carry a risk of transmitting an STI. Each STI has a different risk and severity.
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Parents receive vouchers amounting to the sum that the state spends on their child's education under the concept of school choice. School choice is present in the United States, where an array of alternative options are provided to parents as substitutes to the public school that are present in their district. Options received under school choice include home schooling, virtual schooling and magnet schooling.