an investigator is studying the relationship between the physical and intellectual growth of primary schoolchildren (grades 1-6). within each grade level, she notes that the correlation between the height of children and the size of their vocabulary is zero. when pooling all six grades together, the correlation of height and vocabulary size is likely to be which of the following? (hint: draw a picture; we know that, on average, children get taller from grade 1 to grade 6).