In the first half of the 19th century, why did the u. s. congress work to keep a balance between the number of slaveholding states and free states?

a.) d-day invasion of normandy.

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aug 15, 2012 - although van leeuwenhoek was not the inventor of the microscope, he advanced it more he loved to grind and focus a new lens in order to see the unseen world. most of leeuwenhoek's microscopes were silver. in a simple but significant experiment (1668), he demonstrated that worms found on