During the 18th century .5 billion grams of gold

[mohan.padmini]

http://www.postimage.org/image.php?v=gxhb4zJ

Hello,

Please use the link to view the image.

I have not understood the explanation given.

How is 'are estimated ' present tense?

Plz explain the 1st 3rd and 4th options.

Thanks!

[Ben Ku]

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