What is this paragraph's topic sentence?

Question

In 1867, the United States bought Alaska from Russia for about $7 million. Many Americans thought it was a foolish purchase. They thought Alaska was a wasteland of ice and snow. However, Alaska is rich in oil, fish, and lumber. The purchase of Alaska was a true bargain.
In 1867, the United States bought Alaska from Russia for about $7 million.
However, Alaska is rich in oil, fish, and lumber.
The purchase of Alaska was a true bargain.
Many Americans thought it was a foolish purchase.

Answer 1

In 1867, the United States bought Alaska from Russia for about $7 million.