<0, and so 1−c<0, or equivalently c>1 Choice B is incorrect. If c=1, then a−b=a, or b=0. But it is given that b>0, so c=1 cannot be true. Choice C is incorrect. If c=−1, then a−b=−a, or 2a=b. But this equation contradicts the premise that a<0 and b>0, so c=−1 cannot be true. Choice D is incorrect. For example, if c=−2, then a−b=−2a, or 3a=b. But this contradicts the fact that a and b have opposite signs, so c<−1 cannot be true.