http://digfir-published.macmillanusa.com/astro_interactive_exercise/ie_the_drake_equation.html |
Chapter
1. The Drake Equation
| 51413a45b11c0f181b000023 |
http://digfir-published.macmillanusa.com/astro_interactive_exercise/ie_the_drake_equation_dlap.xml | DLAP questions | 51413a45b11c0f181b000023 |