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Read the selection adapted from Wikipedia and answer the question that follows.
The cause of thunder has been the subject of centuries of speculation and scientific inquiry. The first recorded theory is attributed to the Greek philosopher Aristotle in the third century BC, who speculated that it was caused by the collision of clouds. Subsequently, numerous other theories have been proposed. By the mid-19th century, the accepted theory was that lightning produced a vacuum.
In the 20th century, a consensus evolved that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel. The temperature inside the lightning channel, measured by spectral analysis, varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about 20,400 K (20,100 °C; 36,300 °F). This heating causes it to expand outward, plowing into the surrounding cooler air at a speed faster than sound would travel in that cooler air. The outward-moving pulse that results is a shockwave, similar in principle to the shockwave formed by an explosion or at the front of a supersonic aircraft.
More recently, the consensus around the cause of the shockwave has been eroded by the observation that measured overpressures in simulated lightning are greater than could be achieved by the amount of heating found. Alternative proposals rely on electrodynamic effects of the massive current acting on the plasma in the bolt of lightning. This shockwave is sufficient to cause injury, such as internal contusion, to individuals nearby.
Which sentence helped you find the main idea of the passage?
The cause of thunder has been the subject of centuries of speculation and scientific inquiry.
By the mid-19th century, the accepted theory was that lightning produced a vacuum.
This shockwave is sufficient to cause injury, such as internal contusion, to individuals nearby.
Alternative proposals rely on electrodynamic effects of the massive current acting on the plasma in the bolt of lightning.