Generally speaking, the eastern American cities are (more) ______ than the western ones.

单项选择题One of the most promising methods of storing hydrogen fuel is ______.

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