Answer:
C). When
D). Because
Step-by-step explanation:
'Signal words' are described as the words offering hints or cues to the readers regarding the upcoming event/incidence. It helps them in understanding the text more effectively by showing the cause-effect or compare-contrast relationship.
In the given excerpt, the words 'when' and 'because' that position a cause or a reason leading to a specific effect. By using 'when' the author asserts the cause('ultra-massive star runs out of fuel') leading to the 'initial collapse' and through 'because'("its enormous mass generates such tremendous gravitation"), the outcome of 'no crash' takes place. Thus, options C and D are the correct answers.