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A.They will calm down.Most customers want you to acknowledge that they've been disappointed and want you to express some regret.
If you apology does not include a solution they will be upset
It depends on the reason behind their upset. Some are expecting some kind of compensation, a sincere apology is then not enough
It depends on the customer and the main problem. Some customers accept your apology, and others may try to take advantage of this and want to escalate the issue !