Your friend Gandalf asks if you can give him a ride home for winter break. You agree to the request in part because you know that you will need his help studying for your physics exam. Your agreeing to drive Gandalf home is best explained by

Because so many people use them and say that they are good

Yes it was hope that works

Currently, as of february 2016, an american traveling in france would be by the exchange rate. the weakness of the euro and the strength of the us dollar means that items in france cost less to an american than they otherwise would. of course, if this trend reverses and the us dollar weakens and the euro strengthens, such as was happening in early 2008, an american traveling in france would have an adverse experience because items would cost relatively more.