In his study of pea plants, Gregor Mendel used which method to produce offspring?

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In his study of pea plants, Gregor Mendel used which method to produce offspring?
a.cross-pollination, by using parents that had identical traits
b.cross-pollination, by using parents that had different traits
c.self-pollination, by using one parent
d.random pollination, by using both identical- and different-trait matings