Which word signals an effect?
A. if
B. since
C. because
D. therefore

Therefore.

Explanation:

Use an example and input these words into it. Say, the sun came out so the the flowers bloomed. The sun came out if the flowers came out? No. This could be used to say, if the sun came out the flowers would bloom. But, this isn't a effect sentence. The sun came out since the flowers came out? No. The sun came out because the flowers bloomed? Also no. This would work if you where signaling a cause, the flowers bloomed because the sun came out. So, this leaves therefore. The sun came out, and therefore the flowers bloomed.

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