7. Among the four real numbers $a, b, c, d$, the sum of any three numbers is not less than the other one. Which of the following judgments about these four numbers is definitely incorrect? ( )
(A) There is more than one non-zero number
(B) All four numbers are positive
(C) All four numbers are negative
(D) One of the four numbers is negative
Answer: (C)
Solution: According to the problem, we have
$$
a+b+c \geq d, a+b+d \geq c,
$$

Adding the two equations, we get
$$
a+b \geq 0 \text { . }
$$

Similarly,
$$
a+c \geq 0, a+d \geq 0, b+c \geq 0, b+d \geq 0, c+d \geq 0,
$$

Therefore, it is impossible to have two or more negative numbers.
Hence, the answer is (C).