Consider the sentences below:
1. Global warming ___________ by the greenhouse effect.
2. When ice in the poles melt, sea levels ____________.
3. Carbon dioxide __________ the sun's heat.
4. An American scientist ___________ holes in the ozone layer.
Choose the alternative which presents the correct verb forms that complete the above sentences, respectively:
causes / raises / trap / discovered
is caused / rises / trap / discovered
is caused / rise / traps / has discovered
causes / raise / is trapped / discovered
is caused / raise / traps / has discovered
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