Construct separate data frames for houses sold in Ames, IA, and houses sold in Berkeley, CA. Give the line or lines of R code used to do this. How many houses in the sample are contained in each city?

a) ames_df <- subset(houses_df, city == "Ames"), 75 houses in Ames
b) berkeley_df <- subset(houses_df, city == "Berkeley"), 50 houses in Berkeley
c) ames_df <- filter(houses_df, city == "Ames"), 75 houses in Ames
d) berkeley_df <- filter(houses_df, city == "Berkeley"), 50 houses in Berkeley