Test your metal or mettle?

Which is correct?
Keeping data flowing efficiently across an enterprise WAN tests the _______ of any system administrator.
a. mettle
b. metal

Explanation:
Because these two words are homophones — words that sound identical — many people don’t realize they are actually two different words. However, mettle means the stuff one is made of, the strength and quality of one’s character. Metal, as you probably know, is any of a variety of handy substances used to make cars, computer parts, appliances and so on.

The Guide to Grammar and Writing includes mettle and metal in its Notorious Confusables section.

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