下列现象中,由地球自转引起的是( )
A:昼夜交替
B:四季变化
C:昼夜长短变化
D:正午太阳高度的变化
____+45=____+55,这里运用了加法____,用字母表示是____.
In Sweden, people don't need to _______ too much money on presents, instead, making a meal is enough.