Practice: Matrix Operations

Given the matrices below, which of the following statements is/are defined?
A=[abcd], B=[efghijkl], C=[mnopqrst]A=\left[\begin{array}{c} a&b\\c&d \end{array}\right], \ B=\left[\begin{array}{c} e&f&g&h\\i&j&k&l \end{array}\right],\ C=\left[\begin{array}{c} m&n\\o&p\\q&r\\s&t \end{array}\right]
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