The general tone of the passage is one of ______.

A．Waiters and waitresses usually get low wages.
B．A $ 3.00 tip is usually expected for a meal of $ 20. 00.
C．Waiters and waitresses often get best tips from customers they work most for.
D．Good tipping can help customers get quick service.

A．How canals are constructed.
B．Common types of canal boats and barges.
C．The world's largest canals.
D．How canals are used and classified.