Using BY to indicate how something is done
Q1. We travelled to London ___ train
BY indicates the method of travel. No article is used. One travels BY train, not BY the train.
Q2. She learned French ___ watching films
BY indicates the method. Watching films facilitated learning. Subtitles may have helped.
Q3. He makes a living ___ selling cars
BY shows how something is accomplished. Selling cars provides income.
Q4. Please pay ___ credit card
BY indicates the payment method. Credit cards facilitate transactions.
Q5. The message was sent ___ email
BY indicates the means of communication. Email was the vehicle. Delivery confirmed.