A verb form question tests whether the blank needs a main verb or a form that only describes. Find the subject, then sort the forms by whether they can say what it does or is on their own. Read to the period, because the main verb may come later. Count main verbs in the blank's clause, the group with its own subject and verb. Each clause needs a main verb, but another one needs a joining word rather than sitting beside the first.
Hints
- Hint 1
Read the second sentence all the way to the period, looking for the verb that goes with the rooftop sensors.
- Hint 2
Set the blank and the details about calls aside for a moment. Do the words that remain already form a full sentence?
Step-by-step
Sort the forms and count main verbs
Step 1Find the subject and sort the forms
Start with The rooftop sensors: these sensors are the subject, the thing the second sentence is about. What jobs can the verb forms do? Had recorded and record can be main verbs, which say what a subject does or is, while an -ing form can't be one on its own. Recorded can be a main verb or describe something done to a noun, so we'll check the rest of the sentence to decide.
- Step 2
Read past the blank
Read past ultrasonic calls at frequencies beyond human hearing to allowed her team to compare bat activity. What allowed the team to compare bat activity? The sensors did, so they already have a main verb. The blank has the same subject, and no word such as and joins it to the later verb. That rules out had recorded and record, because each would add a second main verb without a joining word.
- Step 3
Test how the remaining forms work
Can recorded describe the sensors? Use the was test: put was or were before an -ed form to see whether it means something was done to the noun. Sensors were recorded ultrasonic calls doesn't make sense, because the sensors capture the ultrasonic calls. So recorded is another main verb here, while recording describes the sensors without adding one. Choice D.