4. The number of students in Xiaoming's class is less than 40, but more than 30. Therefore, the ratio of boys to girls in the class cannot be ( ).
A. $2: 3$
B. $3: 4$
C. $4: 5$
D. $3: 7$
【Answer】D
【Analysis】The shares corresponding to the four options are: $5,7,9,10$. The fourth 10 cannot divide any number greater than 30 and less than 40, so choose D.