A、In an envelope. B、By a phone call. C、In an email. D、Via an instant message. C

(1)The authorities in Los Angeles have defended their decision to close all public schools for the day because of a suspected security threat. A similar threat was received in New York where the authority said it was not credible, and the city’s police chief called the Los Angeles’ closure a significant overreaction. Peter Bowes reports. (2)"Officials say they decided to close down the entire public school system out of what they called an abundance of caution after they received a threat in an email. The authority said they would search every school in the district to make sure they were safe for pupils to return. As that got underway, New York City officials confirmed they had received the same threat, but quickly dismissed it. A member of the House Select Committee on intelligence has said the threat is believed to be a hoax."
1. Why did the authorities in Los Angeles decide to close all public schools?
2. How was the threat delivered?