问题
单项选择题
For any numbers w and z, w·z=w3z(8-w2). If both z and w·z are positive numbers, which of the following could be a value of w ？（）
A. 9
B. 3
C. 0
D.-2
E.-9
答案
参考答案：E
解析：
The most direct approach here is probably to plug in the answers and see which one gives a positive value for w·z. A and B are out because (8-w2) results in a negative number while w3z is positive, C is out because 0 is not a positive number, D is out because w3z is negative while (8-w2) is positive; only E meets the conditions, because both (8-w2) and w3z are negative, yielding a positive value for w·z.