Read the text and answer the following questions!

    Once upon a time Sammy and his sister Marie visited their grandparents at their farm. Sammy picked up a slingshot to play with out in the woods. He practiced in the woods but he could never hit the target. Getting a little discouraged, he headed back for the lunch. As he was walking back he saw Grandma's pet duck.

    Just out of impulse, he let the slingshot fly which hit the duck square in the head and killed it. He was shocked and grieved. In a panic, he hid the dead duck in the pile of woods. Marie had seen it all from the window but she said nothing. After the lunch, the next day Grandma said, Marie, let's wash the dishes." Marie said, "But Grandma, Sammy told me he wanted to help in the kitchen."

    Then she whispered to Sammy, "Remember the duck?" Sammy got nervous knowing his sister knew about the duck and if he doesn't obey, she may tell his grandparents about it. So without saying anything he did the dishes.

    Later that day, Grandpa asked if the children wanted to go fishing but Grandma said, "T'm sorry but I need Marie to help make a supper." Marie just smiled and said, "Well that's all right because Sammy told me he wanted to help."

    She whispered again to Johnny, "Remember the duck?" So Marie went fishing and Sammy stayed to help. After several days of Marie taking advantage of Sammy, he finally couldn't stand it any longer. He came to Grandma and confessed that he had killed the duck.

    Grandma knelt down, gave him a hug and said, "Sweetheart, I know. I was standing at the window and I saw the whole thing but because I love you, I forgave you. I was just wondering how long you would let Marie take advantage of you."

(Source: http://www.moralstories.org)

Why didn't Sammy tell to his grandmother that he had killed the duck? Because ____