单选题 .  ①Recent studies of the gender gap in the history of United States policies tend to focus on candidate choice rather than on registration and turnout. ②This shift in focus from gender inequality in political participation may be due to the finding in several studies of voting behavior in the United States that since 1980, differences in rates of registration and voting between men and women are not statistically significant after controlling for traditional predictors of participation. ③However, Fullerton and Stern argue that researchers have overlooked the substantial gender gap in registration and voting in the South. ④While the gender gap in participation virtually disappeared outside the South by the 1950s, substantial gender differences persisted in the South throughout the 1950s and 1960s, only beginning to decline in 1970s.22.   Select the sentence in the passage that offers a possible explanation for a trend.