Respond to:
* Means to give a reaction or answer to something. For example:
* "I responded to her email with a polite message."
* "The company responded to the customer's complaint."
At:
* Indicates a place, time, or direction. For example:
* "I arrived at the airport early."
* "Let's meet at 2 pm."
* "He walked at a brisk pace."
To determine which preposition is correct, you need to analyze the context of the sentence and understand the meaning you want to convey.
For example:
* "He responded ___ the questions." Here, "responded" means "gave answers." So the correct preposition is "to": "He responded to the questions."
* "He arrived ___ the airport." Here, "at" indicates a place. "He arrived at the airport."
If you're unsure, think about the meaning of the sentence and which preposition makes the most sense.