Here's why:
* Passive voice focuses on the action being done to the subject, not the doer of the action.
Let's break down the sentences:
* a) The rain continued until dawn. This is in the active voice. "Rain" is the subject, and it's performing the action of "continuing."
* b) John searched for a job. This is also in the active voice. "John" is the subject, and he's performing the action of "searching."
* c) Tree was planted by Mary. This is in the passive voice. "Tree" is the subject, and it's having the action of "planted" done to it. "By Mary" tells us who did the planting.
* d) Lies destroy friendships. This is in the active voice. "Lies" are the subject, and they are performing the action of "destroying."