What proposal, in modified form, became the basis for the Constitution

James Madison's Virginia Plan

Explanation:

At the constitutional convention of 1787 James Madison proposed the "Virginia Plan" which, in its most basic form called for a strong central government with a Bi-cameral legislature that had representatives proportionally based on population.

one of the main reasons was definitely slavery. back in 1861, republican abraham lincoln was elected and ended up being a major influence in the slavery business. slavery greatly affected the texas economy because back in 1845 during the annexation of texas, slaves became a big part of the texas population. many texans that were in favor of the secession influenced groups through party ties.

a.) are both appointed for life