Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 20 to 26.

Do you know the indispensable part of your computer that is more than a century old? The modern typewriter was born in the back of Kleinsteuber's Machine Shop in Milwaukee. That's where Christopher Sholes took some piano wire and a telegraph key and built a crude typing device in 1868. It could type only one letter, a rather fuzzy- looking w, but it was still pretty amazing for its time. Sholes and his partners designed a more ambitious model with all the letters in the alphabet.

The typewriter had a problem, though. Try to type quickly on it, and the type bars banged into one another and got stuck. The solution to that problem resulted in the keyboard we know today.

Sholes consulted with an educator who helped him analyze the most common pairings of letters in the English language. He then split up those letters so that their type bars were farther apart and less likely to jam. That in turn dictated the layout of the keyboard - known as QWERTY, for the first five letters in the upper row. In a manner of speaking, he slowed down the typists to prevent jamming, and thus speed the typing.

In 1873 the Sholes & Glidden Type Writer became the first to be mass-produced, and its keyboard layout was soon standard on all typewriters. The original Sholes & Glidden typed only capitals and was designed in an awkward fashion that prevented the typist from looking at the paper while typing. Ads trumpeted its value to clergymen and lawyers, but it sold quite poorly at first. One reason: it was expensive. It cost $125, the equivalent of more than $1,700 today.

What was the weak point of the first typewriter model?