If you have multiple dropdowns in a single question but do not want students to receive partial credit if they only get some of the dropdowns correct, you will need to specify that in the XML of the question itself.
Step 1: When in the editing portion of the question, click Advanced Editor to open the question in XML.
Step 2: In the XML, add this XML tag below <problem>: <optionresponse partial_credit="false">. If the question shows more than one "optionresponse," delete any others so the only one is right below <problem>. Be sure to close the tag at the end of the xml with </optionresponse>
Step 3: The final "all-or-nothing" grading will look like this.