Which of the following correctly states the number of comets that scientists estimate to be located in the oort cloud and the number that have actually been observed in the oort cloud (with telescopes)?
a) estimated number in oort cloud: 1 trillion observed in oort cloud: zero
b) estimated number in oort cloud: 1 billion observed in oort cloud: 10,000
c) estimated number in oort cloud: 100,000 observed in oort cloud: 50
d) estimated number in oort cloud: 1 million observed in oort cloud: 1,000