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Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you to remember: You have this great ice cream... that is, until your cone (こな) turns to powder, and your ice cream falls to the ground. A sonic boom powdered your ice cream cone, and left you very messy.
You can also use こ for this kanji. Just imagine that the powder breaks down even further, and all that is left of the reading is こ. This reading feels a bit older, but it's still a-okay.
I’ve been eating too many flour-based foods lately.