It's useful always to carry small change for things like exact fare buses, but do not carry large sums of cash. Instead keep the bulk of your money in travellers' cheques which Can be purchased both in the US and abroad and should be in dollar denominations. The best known cheques are those of American Express, so you will have the least difficulty cashing these, even in out of the way places. Thomas Cook travellers' cheques are also acceptable, especially as lost ones can be reclaimed at Some car rental companies. Dollar denomination cheques can be used like regular money. There's no need to cash them at a bank: use them instead to pay for meals, supermarket purchases or whatever. Ten or twenty dollar cheques are accepted like this almost always and you'll be given change just as though you'd presented the cashier with dollar bills. Be prepared to show I. D. when you cash your cheques.
Credit cards can be even more valuable than travellers' cheques, as they are often used to guarantee room reservations over the phone and are accepted in lieu of deposit when renting a car--indeed without a credit card you may be considered so untrustworthy that not only a deposit but your passport will be held as security too. The major credit cards are VISA, Master Charge and Access, Diners Club and American Express. If you hold a bank card, it could well be worthwhile to increase your credit limit for travel purposes--you should ask your bank manager.
Why is it useful to carry enough 25-cent coins with you？

A. They can act as small change for the exact bus fares.
B. There is generally no problem in using them in Canada.
C. As a basic unit of money, they are gradually gaining in usage.
D. They may come in handy for pay phones or laundry machines.