I found following sentence from one of BBC articles.
What I cannot understand is why they have used "are" before "a powerful tool", though "a powerful tool" is a singular phrase. Therefore, I would really grateful if someone could provide me with an explanation about the construction.
Radar images can be difficult to interpret for the casual observer, but they're a powerful tool with which to monitor and map the Earth.
Reference
The author is thinking of the sentence this way. eg Radar images can be difficult to interpret for the casual observer, but the use of radar imagery is a powerful tool with which to monitor and map the Earth. Clive
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The author is thinking of the sentence this way.
eg Radar images can be difficult to interpret for the casual observer, but the use of radar imagery is a powerful tool with which to monitor and map the Earth.
Clive
dileepathey're a powerful tool with which to monitor and map the Earth.
Radar images are a powerful tool with which to monitor and map the Earth. (The subject is plural. The verb agrees with the plural subject.)