简述计算机网络安全的定义。
参考答案：
计算机网络安全是指利用管理控制和技术措施，保证在一个网络环境里，信息数据的机密性、完整性及可使用性受到保护。
董事不得同本公司订立合同或者进行交易。（）
Woman: Have you heard about the plane crash yesterday It caused 120 deaths. I’m never at ease when taking a flight. Man: Though we often hear about air crashes and serious casualties, flying is still one of the safest ways to travel. Question: What do we learn from this conversation（）。
A. The man thinks travelling by air is quite safe.
B. The woman never travels by plane.
C. Both speakers feel nervous when flying.
D. The speakers feel sad about the serious loss of life.
