One way to demonstrate assertiveness while interacting with a customer is to

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One way to demonstrate assertiveness while interacting with a customer is to avoid expressing disagreement with the customer. This will prevent relationship breakdown.

Explanation:

A "quality customer service" is one which establishes rapport with customers. Besides this, it should also be fast and efficient so that problems encountered by customers will be addressed on time. This will allow them to continue using the service or purchasing the products which the company is providing.

"Assertiveness" is an important quality that a customer service provider must possess. It should not be mistaken for aggressiveness. Assertiveness allows the customer service provider to find a proper solution to the customer's problems or needs. Thus, he should avoid expressing disagreement with the customer by providing solutions. Giving the customer an alternative option makes them feel that you are listening to their problems and issues. It also makes the conversation more constructive.

So, this explains the answer.