Read the following passage and mark the letter A, B, C, or D to indicate the correct word or phrase that best fits each of the numbered blanks. 

     All living things require energy to do the work necessary for survival and reproduction. But what is energy? Energy is simply the ability to do work, (39) ________. work is done when a force moves an object. Let's consider your own needs for a moment. You need energy to turn on and turn off your computer. You need energy to (40) _______ of bed in the morning. And, yes, you need energy to reproduce. So where does energy come from and how do we use it? On Earth, energy ultimately comes from the sun. Plants use the sun's energy to make sugar. Organisms, in turn, use sugar as a (41) ________ of energy to do work. Plants use energy from sunlight to make sugar and oxygen from carbon dioxide and water. The process by which carbon dioxide and water are (42) _______ to sugar and oxygen using sunlight is referred to as photosynthesis. This is an endergonic reaction, meaning energy is required by the reaction. Specifically, energy is required to put the carbon dioxide and the water molecules together to form sugar. Sun (43) _______ the energy needed to drive photosynthesis, and some of the energy used to make the sugar is stored in the sugar molecule. 

But what is energy? Energy is simply the ability to do work, (39) ________.