When the prairies were being settled, undoubtedly, the good economic conditions of the 1950's supported a growth in the population, but the expansion also derived from a trend toward earlier marriages and an increase in the average size of families
句意：毫无疑问，当人们在草原上定居时，20世纪50年代良好的经济状况支撑了人口的增长，但这种增长也源于早婚和家庭平均规模的增加。trend趋势。tendency趋向。二者意思最接近。aim目标。growth增长。directive指令。因此，本题的正确答案为 A。