Which of the Following Phrases Describes a Manipulated Variable

Multiple Choice

Which of the following phrases describes a manipulated variable?

A) "Participants wrote down how many text messages they had sent the day before."
B) "Researchers recorded whether participants volunteered to help the student in need or did not volunteer."
C) "Participants were placed in the high tempo music condition, the low tempo music condition, or the no music condition based on which color card they randomly drew from a deck."
D) "Researchers recorded the length of time participants took to complete the anagrams."