<p>In today’s question, “handcuffs” is plural? I would think it would be singular because it’s one pair of handcuffs.
Ex) I saw that pair of pants in the store.</p>
<p>Take away the word “pair” and your example becomes “I saw those pants in the store.” Likewise with handcuffs- “The handcuffs require…” vs. “The pair of handcuffs requires…”.</p>
<p>What I’m confused about is… shouldn’t it be “they require keys to open” not “they require a key to open”?</p>
<p>Think of the second part of the sentence as referring to “two metal rings.” The question is still referring to a single pair of handcuffs, which requires a single key to open.</p>