hard to come up (46)______ alternative fuels which can replace

Câu hỏi :

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 46 to 50.

We all want to live in a clean and green world and breathe pollution free air. For this kind of environment we desperately need a fossil fuel free world. Scientists are toiling hard to come up (46)______ alternative fuels which can replace conventional fuels. One such study was presented at the 237th National Meeting of the American Chemical Society. This study throws interesting light on the first economical, eco-friendly process to (47)______ algae oil into biodiesel fuel. The scientists are quite hopeful that one day America will become independent (48)______ fossil fuels. Ben Wen is the (49)______ researcher and vice president of United Environment and Energy LLC, Horseheads, N.Y. According to him, “This is the first economical way to produce biodiesel from algae oil. It costs much less than conventional processes because you would need a much smaller factory, there (50)______ no water disposal costs, and the process is considerably faster.”