The agent had disclosed the secret before it was evening. 

1. The secret was disclosed by the agent before it was evening. 
2. The secret had disclosed by the agent before it had been evening. 
3. The secret had been disclosed by the agent before it was evening.
4. The secret was disclosed by the agent before it had been evening. 

Explanation

Explanation:
The correct answer is option -C
The given sentence is the active voice
We have to changed it into passive voice.
Active voice- Subject +verb+object +complement.
Passive voice- object + auxiliary verb+V3+by+Subject+complement.
The given sentence is in the past perfect tense.
Only option C is grammatically correct.
Option A-it is in simple past form.But given sentence is in past perfect tense.
Option -B - it is in the present perfect tense.But usage of `been' is on wrong place.it should be after helping verb `Had'.
Option D- It is in the simple past form.
So except option C all are grammatically incorrect.

Surely the lost child must have been found by now. 

1. Surely must have found the lost child by now. 
2. Surely someone must have found the lost child by now. 
3. Surely now must have found the lost child 
4. Now must have found the lost child surely.

Explanation

Explanation:
The correct answer is option number -B
The given construction is in passive form
We need to changed it in the a active voice.
The given sentence is in present perfect tense.
So option B is the correct answer.
Rest options are grammatically incorrect.

We serve hot meals till 10.30 guests can order, coffee and sandwiches upto 11.30. 

1. Hot meals are serving till 10.30 ; coffee and sandwiches are ordering by guests till 11.30. 
2. Hot meals are being served till 10.30 coffee and sandwiches are being ordered till 11-30. 
3. Hot meals are served till 10.30; coffee and sandwiches may be ordered till 11.30. 
4.  Hot meals will be served till 10.30; coffee and sandwiches will be ordered upto 11.30. 

Explanation

Explanation: The correct answer is option number -C

The given sentence is in active voice.

Lie facedown; stretch your arms in front. 

1. You are face down, arms are to be outstretched
2. You should be lying face down, with arms outstretched. 
3. You should be lying face down; let arms stretch out. 
4. Let face be down; let arms be stretched out. 

Explanation

Explanation:
The correct answer is option number -D
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If the sentence starts with `verb' then we use `let' while changing it into passive voice.
Given construction - verb +object.
Passive voice - Let + object + be+ V3.
Remaining options are incorrect.

By: Parvesh Mehta