Who does Briony think the housekeeper's son is attacking?  Answer the above question based on the context below:  In 1935 England, Briony Tallis is a 13-year-old from a wealthy family. She has just completed writing her first play to mark her brother's homecoming and plans to stage it later that day with her visiting cousins. Looking out of her bedroom window, she spies on her older sister, Cecilia, and the housekeeper's son, Robbie Turner, on whom Briony has a crush. Cecilia is undressing and dips into the fountain pool; a moment later, she climbs out, her undergarments wet, all while Robbie watches. Cecilia had gone to the pond to fill a vase, Robbie grabbed one of the handles, and it broke. A part fell into the pond, and Cecilia jumped in to retrieve it, but to Briony, it looked as if Robbie had ordered Cecilia to undress and go under the water. Robbie drafts a series of notes to Cecilia apologizing for the incident, namely breaking the vase and laughing about it. One contains an explicit expression of his sexual desire for her, including frequent and crude usage of the word "cunt": he writes it only as a joke, and it makes him laugh to himself. He writes another, more formal letter, and asks Briony to deliver it. Only after she has gone does he realise he has given her the explicit letter. Briony reads the letter before giving it to Cecilia. Later, she describes it to her older visiting cousin, Lola, who calls Robbie a "sex maniac". Paul Marshall, a visiting friend of Briony's older brother's and a chocolate magnate, introduces himself to the visiting cousins and appears to be attracted to Lola. Before dinner, Robbie apologises for the obscene letter, but Cecilia surprises him and confesses her secret love for him. They then proceed to make passionate love in the library when Briony walks in, and thinks that Cecilia is under attack. Cecilia and Robbie try to pass the incident off.
A:
Cecilia