The half-life of carbon-14 is 5,700 years, meaning that after this period, half of the original amount of carbon-14 will remain.
If the sample of wood originally has 20 grams of carbon-14 when it is alive, after 5,700 years (one half-life), it will have:
20 grams / 2 = 10 grams of carbon-14.
So, the correct response is:
10 grams