When only one entity of an entity set may participate in a relationship, we express this by connecting the entity set with the relationship by an arrowed line

Connect the Customer entity set with the CustAcc relationship set with an arrowed line.

1. Select the "Connection" tool and connect the Customer entity set with the CustAcc relationship set .
2. Move the mouse over the line, and press the mouse button for a second until the popup menu opens.
3. Choose the menu item "Cardinality > 1". This turns the connection into an arrowed line

Click at the "Compare with Solution" button to see if your diagram is correct

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Source: http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node8.html