①血缘关系的结合 ②工具简陋粗糙 ③人们头脑简单 ④生活环境险恶
A:①②
B:③④
C:①③
D:②④
执行如图所示的程序框图,若输入n的值为7,则输出的s的值为( )
In the UK, this is the ground floor. In the US, this is the first floor.