问题如下：
A report on long-term stock returns focused exclusively on all currently publicly traded firms in an industry is most likely susceptible to:
选项：
A. look-ahead bias.
B. survivorship bias.
C. intergenerational data mining.
解释：
B is correct.
A report that uses a current list of stocks does not account for firms that failed, merged, or otherwise disappeared from the public equity market in previous years. As a consequence, the report is biased. This type of bias is known as survivorship bias.
请问A是什么意思呢？一直都没弄明白