Fill in the Blanks past perfect exerciseOnline version exercises of past perfect by Cristhian Camilo Ruiz Gil 1 thought read learned were known had had had children's were was decided contacted been Twin World by Cory Daniels I had about including my twins in a study until I about one woman's experience . Ana had interested in the study of twins all her life . Ana was a twin herself , and she twin boys . Before her twins born , she read a lot about twin studies . After she done a little research , Ana found an early reading study for twins . It sounded like a useful study . It looked at the role of genetics and environment in developing reading skills . She the researchers and learned that she had to wait until the twins were four years old . When she enrolled the twins in the study , she hadn't that the twins needed to give a DNA sample to prove they twins . As soon as Ana this , she took the twins out of the study . Ana thought that taking a DNA sample an invasion of her privacy . When I read about her experience , I not to put my twins in a study after all .