问题如下：
Factor (smart beta) strategy ETFs are least likely to be used by investors:
选项：
A.to modify portfolio risk.
for tactical trading purposes
to seek outperformance versus a benchmark.
解释：
B is correct. Factor strategy ETFs are usually benchmarked to an index created with predefined rules for screening and/or weighting stock holdings and are considered longer-term, buy-and-hold investment options rather than tactical trading instruments. The strategy index rules are structured around return drivers or factors, such as value, dividend yield, earnings or dividend growth, quality, stock volatility, or momentum. Investors using factor-based investing seek outperformance versus a benchmark or portfolio risk modification.
请问smart beta是什么？在哪个地方呢？
