【正确答案】 C

【答案解析】By the triangle inequality (KEY FACTS J12 and J13),
· The third side must be less than 9 + 10 = 19. (Ⅲ is false.)
· The third side must be greater than 10 -9 = 1. (Ⅰ is false.)
· Any number between 1 and 19 could be the length of the third side. (Ⅱ is true.) The answer is C.

【正确答案】 D

【答案解析】Check each choice. (A) If x = 5, [*]= 2. (B) This one is more difficult; the only possibility if x = 1, in which case [*] = 0. If you don't see that immediately, keep Choice B under consideration, and test the rest. (C) If x = 1, [*] = 1. (D) For positive x, [*] is always greater than 1 but less than 2; [*] cannot be an integer—that's it. If you didn't reason that out, check Choice E. [*] is an integer if x = 6. You should have eliminated at least Choices, A, C, and E.