Why were the first light bulbs impractical?

Câu hỏi: Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer for each of the questions from 35 to 42.
The light bulb changed human existence by illuminating the night and making human activity possible in darkness. The electric light, one of the everyday conveniences that most affects our lives, was invented in 1897 by both Thomas Alva Edison in the United States, and Sir Joseph Wilson Swan in England at the same time
However, the story of the electric light actually goes back to 1811, when Sir Humphrey Davy discovered that an electrical arc passed between two poles produced light. In 1841, experimental lights were installed as public lighting along the Place de la Concorde in Paris.
Other experiments were undertaken in Europe and America, but the arc light eventually proved impractical because it burned out too quickly. Inventors continued to work on the problem of developing a reliable electric light that would be practical for both home and public use instead of gas lights.
The solution lay not in an electrical arc in open space, but in electricity passed through a filament. The breakthrough theory became known as the Joule effect after James Prescott Joule. He theorized that electrical current, if passed through a resistant conductor, would glow white-hot with heat energy and thus produce light.
Edison decided to try a carbonized cotton thread filament. When voltage was applied to the completed bulb, it gave off a soft orange glow. Just about fifteen hours later, the filament finally burned out. Further experimentation produced filaments that could burn and would last for 1500 hours, and he began to market his new invention.
Why were the first light bulbs impractical?
A. They lasted only a short time.
B. Glass was too expensive for light bulbs.
C. Electricity wasn't widely available.
D. They were too expensive.