Multiple choice

Rubber__from vulcanized silicones with a high molecular weight is difficult to distinguish ....

  1. is produced

  2. that produces

  3. producing

  4. produced

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The sentence already has a main verb ("is difficult"). Therefore, the blank needs a participle to modify the noun "Rubber" in a reduced relative clause. The past participle "produced" correctly expresses a passive meaning ("rubber which is produced"), whereas "producing" would imply an active meaning.