The line cord of a vacuum cleaner must be of the
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 A. grounded type.
 B. zip type.
 C. jacketed type.
 D. cotton-covered type.
To determine which type of cord a vacuum cleaner should have, we need to understand the requirements for electrical safety.
A. The grounded type is the correct answer. In order to prevent electrical shocks, it is important for a vacuum cleaner to be grounded. This means that the electrical cord should have a third prong for the grounding wire, which helps to safely discharge any excess electrical current.
B. The zip type is not a valid option for a vacuum cleaner's cord. This term is typically associated with fasteners and has no relevance to electrical cords.
C. The jacketed type is a general term that refers to cords that are covered in a protective outer layer, which is a common feature for most electrical cords. While it is important for the cord to be jacketed, this does not address the grounding requirement specified in the question.
D. The cotton-covered type is also not a suitable option for a vacuum cleaner's cord. Cotton covering is not commonly used as insulation for electrical cords because it presents a higher risk of fire hazards.
In conclusion, the correct answer is A. The line cord of a vacuum cleaner must be of the grounded type. To determine this, you can ensure that the cord has a third prong for grounding as a safety measure.