Please choose the correct answer to the question: A student sets a block at the top of a wooden ramp. The student pushes the block. As the block slides down the ramp, it slows down and then comes to a stop. What force causes the block to slow down?

Answer1: the force of gravity Answer2: the force of friction Answer3: the force of magnetism Answer4: the force of the student's push

Answer format: answer1/answer2/answer3/answer4