sentences: coal is a kind of fossil fuel & gasoline is a kind of fossil fuel ->  coal and gasoline are kinds of fossil fuels  coal and gasoline are kinds of fossil fuels & burning fossil fuel is a kind of human activity ->  burning fossil fuels such as coal and gasoline is a kind of human activity  burning coal emits sulfur dioxide & burning oil emits sulfur dioxide ->  burning coal and oil emit sulfur dioxide  sulfur dioxide is a kind of gas & emitting sulfur dioxide causes acid rain ->  sulfur dioxide is a kind of gas that causes acid rain when emitted  burning coal and oil emit sulfur dioxide & sulfur dioxide is a kind of gas that causes acid rain when emitted ->  gasses from burning coal and oil cause acid rain  gasses from burning coal and oil cause acid rain & oil is a source of gasoline ->  gases from burning coal and gasoline cause acid rain  gases from burning coal and gasoline cause acid rain & if something causes something else then that something contributes to the formation of that something else ->  gasses from burning coal and gasoline contribute to the formation of acid rain  burning fossil fuels such as coal and gasoline is a kind of human activity & gasses from burning coal and gasoline contribute to the formation of acid rain -> burning fossil fuels such as coal and gasoline is a kind of human activity that contributes to the formation of acid rain