The thief was observed ____ things in the supermarket and was caught on the spot

选C,observe sb.doing sth.发现某人做某事,被动 sb.is observed doing sth.
另有词组：observe sb.do sth.表示经常看到某人做什么事,被动为 sb.is observed to do sth.observe 表被动时，to要加，那为什么c（没to）也行？如果主动时是observe sb. do sth，表示观察某人做某事的整个过程，就是指有意识地去观察别人做某事。被动句要加to。如果主动时是observe sb. doing sth， 指无意地，恰好看到某人正在做某事，被动句不用加to。