Read the passage given below and answer the question.

Rusty was a quiet, imaginative child who loved the natural world. He had grown up in the hills with his grandparents and had never known his parents. When his grandparents died, Rusty was sent to live with his father in the city. Rusty struggled to adapt to life in the city, missing the peace and quiet of the hills. He found solace in exploring the city's parks and gardens, and in spending time with his new friend Somi. One day, while exploring a part, Rusty came across a group of monkeys. He was fascinated by their antics and spent hours watching them. As Rusty grew older, he continued to struggle with city life. He found himself yearning for the hills more and more, and eventually decided to return to his grandfather's house. He was surprised to find that the house had been sold, but he was welcomed back by the new owners, a kind couple who had heard about Rusty from his grandfather. Rusty was thrilled to be back in the hills, surrounded by nature once again. He spent his days exploring the forests and meadows, and made friends with the local wildlife. He also began to write stories about his experiences, and his writing talent was soon discovered by a publisher. Rusty went on to become a successful writer, but he never forgot the lessons he had learned in the hills about the importance of nature and the simple pleasure of life.

What happened to Rusty's grandfather's house after his grandparents died ?

The house was sold