Which of these led to slaves becoming more valuable than before? A. The Tariff of Abominations
B. The American System
C. The McCormick reaper
D. The cotton gin

the  seven  days  battles  were  a series of seven battles over seven  days from june 25 to july 1, 1862, near richmond, virginia,  during  the american civil war. the  seven  days ended with mcclellan's army in relative safety next to the james river, having suffered almost 16,000 casualtiesduring  the retreat.

it was the first major victory for the colonists.

i beleive that d is the right answer