Given a scientific passage and an answer, generate a question for the given answer.

Q: Passage: Chlamydia is the most common bacterial STI in the U. S. Females are more likely to develop it than males. Symptoms may include burning during urination and a discharge from the vagina or penis. Answer: chlamydia

A: What is the most common bacterial sti in the u. s.?
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Q: Passage: The mechanical advantage of a machine is the factor by which the machine changes the input force. It equals the ratio of the output force to the input force. A wheel and axle may either increase or decrease the input force, depending on whether the input force is applied to the axle or the wheel. Answer: mechanical advantage

A: What is indicated by the ratio of the output force to the input force in a machine?
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Q: Passage: The scientists used geologic dating techniques on seafloor rocks. They found that the youngest rocks on the seafloor were at the mid-ocean ridges. The rocks get older with distance from the ridge crest. The scientists were surprised to find that the oldest seafloor is less than 180 million years old. This may seem old, but the oldest continental crust is around 4 billion years old. Answer: mid-ocean ridges

A:
Scientists found that the youngest rocks on the seafloor were where?
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