关于礼仪,下列说法正确的有( )①礼仪是一门学问,有特定的要求②随着社会的发展不断变化③礼仪的掌握是自然而然形成的④所有传统的礼仪都要遵守
A:①②
B:②③
C:②④
D:①③
下列函数中周期为π且为偶函数的是( )
Many young people, most of ____were well-educated, headed for remote regions to chase their dream.