Question: If **mdf** is a data frame, which statement is true ?

1. **ncol(mdf)** equals **length(mdf)**.
2. The number of rows must equals the number of columns.
3. The legnth of any column in **mdf** may differ from any other column in **mdf**
4. All columns must have the same data type.

Answer: The correct answer of the above question is Option A:**ncol(mdf)** equals **length(mdf)**.