Answer:
Personal referrals
Explanation:
A referral can be said to be the reference of a person or thing to another due to the abilities and services that the person/thing can offer. Before a referral can take place, the referrer has to have confidence as well as be able to vouch for the person or thing he.she is referring.
In the case of Mary, she is requesting names of friends of customers, businesses, etc that might be interested in her cleaning services from her existing customers.
This means that for her existing customers to be able to give out the information she requests of them, they must have great trust and confidence in her cleaning abilities as evidenced by the works that she has done for them.
Through this information given to Mary, she is able to get more potential clients and her service range continues to expand.
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