C

1003187102

Level: 
C
For \( x \), \( y\in\mathbb{R} \) consider \( |x+y|=|x|+|y| \).
The equality holds if and only if the sign of \( x \) and \( y \) is the same.
The equality does not hold for any \( x \) and \( y \).
The equality holds if and only if \( x \) and \( y \) are all positive.
The equality holds if and only if \( x \) and \( y \) are both nonpositive.