The envelope was thick and heavy, made of yellowish parchment, and the address was written in emerald-green ink. There was no stamp. Turning the envelope over, his hand trembling, Harry saw a purple wax seal bearing a coat of arms; a lion, an eagle, a badger, and a snake surrounding a large letter H.
"Hurry up, boy!" shouted Uncle Vernon from the kitchen. "What are you doing, checking for letter bombs?" He chuckled at his own joke.
Harry went back to the kitchen, still staring at his letter. He handed Uncle Vernon the bill and the postcard, sat down, and slowly began to open the yellow envelope.
Uncle Vernon ripped open the bill, snorted in disgust, and flipped over the postcard. "Marge's ill," he informed Aunt Petunia. "Ate a funny whelk. . ."
"Dad!" said Dudley suddenly. "Dad, Harry's got something!"
Harry was on the point of unfolding his letter, which was written on the same heavy parchment as the envelope, when it was jerked sharply out of his hand by Uncle Vernon. "That's mine!" said Harry, trying to snatch it back.
"Who'd be writing to you?" sneered Uncle Vernon, shaking the letter open with one hand and glancing at it. His face went from red to green faster than a set of traffic lights. And it didn't stop there. Within seconds it was the grayish white of old porridge.
(Harry Potter and the Sorcerer's Stone)
(1) If the sentence were ‘who’d write to you? (A)’, it could imply serial multiplicity of writing. So (A) is not proper for the case indicating just an occurrence. Is this what the original meaning?
(2) If the sentence were ‘who is writing to you? (B)’, it could imply Harry already know who they are. So (B) is not proper, for he doesn’t know who they are. Is this what the original meaning?