单项选择题

Which of the following statements is true of fraternal twins

A．They have the same blood type.
B．They can be of different sexes.
C．They have the same genes.
D．They occur at random in the population.
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单项选择题
Another term for identical twins is__________.
A．fraternal twins
B．mirror twins
C．single-egg twins
D．separate-egg twins
单项选择题
The passage seems to concentrate on __________.
A．how twins are born
B．how twins grow up
C．characteristics of identical twins
D．ways to distinguish twins
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