Question 390
MediumOriginal questionRivers run into the sea all the time, and river water tastes fresh. So why is the sea salty? The answer is that most of the ocean's salt began on land.
Rainwater is very slightly . As it falls on rocks, it slowly wears them away, releasing tiny amounts of minerals. Streams and rivers carry these minerals down to the ocean. River water contains too little of them to taste salty, but the ocean has been collecting them for a very long time. Some are used by sea creatures and removed from the water again; others are not, and they have built up over the .
There is a second source, under the sea itself. Sea water into cracks in the ocean floor, where it is heated by hot rock below. The heat sets off chemical reactions, and the water picks up metals such as iron, zinc and copper from the rocks around it. It then comes back out through openings in the sea floor, carrying these minerals with it.
Undersea volcanoes add a little more, releasing minerals directly into the water when they erupt. Taken together, the salt of the ocean is the slow work of rain, rivers and the fire under the sea floor.
The last sentence describes the ocean's salt as "the slow work of rain, rivers and the fire under the sea floor". This sentence
Show the answer and explanation
Correct answer: A
Explanation
Rain and rivers are the land source in the second paragraph; "the fire under the sea floor" stands for the heated vents and the volcanoes in the third and fourth.
The sentence gathers them together as a summary.
Worth noting
No new source is added; every item in the sentence has already been explained.
- Concept
- Recognising a summarising sentence
- Typical timeTypical
- 45 s
- Your timeYours
- —
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