In this sentence, why is "are" used instead of "is"? I think "key to the relationship" should be followed by "is" because the word "Key" is acting as a singular subject. But in the image it is "are". Help me out; I am confused.
What a difference an article makes!
You are reading the sentence as if it should be:
The key is not just defense and security but also innovation, agriculture.
And indeed, were key preceded by the definite article, you would be correct.
But key here has no article: it's a predicate adjective placed in inverted order because key is, well, the key word in this sentence.
Crucial to the industry are access to viable markets and visibility.
Most important to the deliberations were the chairman's willingness to compromise and an unending supply of good vodka.