19. Natural gas can serve as an _____to coal and oil because it is more environmentally friendly.
A、option
B、extra
C、addition
D、alternative
Directions:There are 15 incomplete sentences in this section. For each sentence there are four choices marked A, B, C and D. Choose one answer that best completes the sentenceand blacken the corresponding letter on the Answer Sheet.6–-Did you find the film boring?一-Not at all. It was _____