Read the telephonic conversation between Rohit and Radhika and answer the question that follows:
Rohit:" Hello!"
Radhika: "Hello, could I speak to Shreya please?"
Rohit: "No, she has gone to the market to purchase balloons for her birthday party. I am her younger brother, Rohit."
Radhika: "Oh! Would you convey my message to her? I am her friend Radhika."
Rohit: "Of course, with pleasure."
Radhika: "Tell her that I will not be able to attend her birthday party because my mother has slipped from the stairs, and I have to take her to the hospital."
Rohit: "Okay, I'll convey your message to Shreya and you do take care of your mother. Bye."
Radhika will not be able to attend Shreya's birthday party because _____