The writer mentions all of the items listed below except （）

单项选择题What' s the reason for twins to be at a disadvantage in their intellectual development（）

A．They can only understand their own private language.
B．They communicate with outsiders less than non-twins.
C．There is little stimulation for them.
D．Adults don' t like to talk with them.

单项选择题Many people don' t know that （）

A．twins understand each other very well
B．twins are slow to learn to talk
C．twins are unlikely to do less well at school than other children
D．there exists more communication between twins