Consider these sentences: Jane spent all night working on the assignment. This is why she was sleepy in class.
How would you rewrite them to make the message clearer?

Jane spent all night working on the assignment, and this is why she was sleepy in class.
Jane spent all night working on the assignment. That is why she was sleepy in class.
Since Jane was sleepy in class, she spent all night working on the assignment.
Since Jane spent all night working on the assignment, she was sleepy in class.

Read the excerpt from president franklin roosevelt's "day of infamy" speech. which sentence shows his use of ethos? japan has, therefore, undertaken a surprise offensive extending throughout the pacific area. the facts of yesterday and today speak for themselves. the people of the united states have already formed their opinions and well understand the implications to the very life and safety of our nation. as commander in chief of the army and navy i have directed that all measures be taken for our defense. but always will our whole nation remember the character of the onslaught against us. no matter how long it may take us to overcome this premeditated invasion, the american people in their righteous might will win through to absolute victory.