A．in gas formB．putting it in liquidC．preserving it in t……

单项选择题Which of the following is the characteristic of hydrogen

单项选择题It can be inferred from the passage that the Sun’s light travels ______.

单项选择题Which sentence proves that setting the giant reflector is a delicate operation

单项选择题What does the passage mainly discuss

单项选择题According to the passage, in the late 1960s some residents of Boston were concerned with which aspect of skyscrapers