Ingenuous or ingenious — what’s the difference?

Which is correct?
The reason spam continues to make money for the people who send it is that a tiny number of recipients are _________ enough to respond to it. The trick is to send out a large enough volume of spam that if even a tiny fraction of recipients respond, it’s lucrative.
a. ingenuous
b. ingenious

Answer: a

Explanation:
Ingenuous means unworldly and naive, possibly to the extent of being a trifle simple. Other than when the word is mistakenly used instead of ingenious, we mostly hear it in its negative form, disingenuous, which means, essentially to be wily but pretending to be unworldly or lacking in knowledge, usually to serve some underhanded purpose.

Ingenious, on the other hand, means clever, inventive.

Simon Kevin writes about these two in Daily Writing Tips. Here’s an excerpt:

“Sometimes a single letter can make a great deal of difference to the meaning of a word. Take, for example, the two words ingenious and ingenuous. Ingenious means clever, original or inventive. It derives ultimately from the Latin word ingenium, which means a natural capacity or talent. It’s the same word from which engine, among other words, derives.”

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