Why are most speakers of American and British English able to understand one another with little difficulty?

Which sentence correctly uses a colon to denote a list or enumeration? a. each group will have members fill the following roles: leader, note-taker, and time-keeper. b. our toughest monthly sales goals are in: january, july, and august. c. marketing, distribution, and customer service: are the focus areas for 2017. d. the company will be opening two new offices in: harrisburg and reading.