问题如下：
There are many potential difficulties in trying to use an external database. Which of the following options is most unlikely to be a limitation of using external databases?
选项：
A. External databases must be carefully checked and used.
B. External databases may have some biases, such as collection biases.
C. External databases can not cover all risk events.
D. Data from external databases cannot be used when calculating company internal risk index.
解释：
D is correct.
考点：Challenges with using external data
解析：在使用外部数据进行分析时，会面临着诸多需要注意的地方，比如说本题中的A、B、C三个选项。选项D是干扰项，因为在计量公司的操作风险时，需要将内部数据与外部数据进行结合，否则只靠公司内部产生的高频低损的数据，无法预测到极端情况，即无法预测具有低频高损特征的数据。
可不可以解释一下？B为什么不是limitation？