What is a half-life? Suppose a radioactive substance has a half-life of 1800 years. What fraction will be left after 1800 years? After 3600 years? After 7200 years? What a half-life? O time required for each doubling in exponential growth is called the half-life B. In the case of exponential decay, the value of the quantity repeatedly decreases to half its value with each halving occurring in a time called the half-life. O C. In case of exponential decay, value of the quantity repeatedly increases to half its value with each halving occurring in a time called the half-life. ODNone of the above. What fraction will be left after 1800 years? Type a simplified fraction) What fraction will be left after 3600 years? (Type simplified fraction. What fraction will be left after 7200 years? Box Type simplified fraction)