In the following passage some of the words have been left out. First read the passage over and try to understand what it is about. Then fill in the blanks with the help of the alternatives given.

The Solar System has been a complicated wonder for the astronomers. This is a ___(1)___ to which we may never have the exact answer. Man has wondered ___(2)___ the age of the Earth ___(3)___ ancient times. There were all kinds of ___(4)___ that seemed to have the ___(5)___. But man could not begin to ___(6)___ about the question scientifically until about 400 years ___(7)___. When it was proved that the ___(8)___ revolved round the Sun and the Earth was a ___(9)___ of our Solar System, then scientists knew where to ___(10)___ .