13．_______ the students in the primary school is about three thousand，and of them are girls．

B
13．B【解析】句意：这所小学的学生数量大约是3，000．其中2/3是女生。由句中的three thousand知道本题要表达的是数量，the numher of指“……的数量一，a number of表“许多”；在分数表达中，若分数的分子超过1，则分母应变复数，2/3应表达为two thirds。