What is the need for using Test.StartTest() and Test.StopTest() Methods ?

What should be done in a visualforce page to Override the Standard view button of a standard object?

In a before insert trigger on a custom object A, we are checking if the email field is empty or not.
If not then we are checking if a contact with same email exists or not. The code fails when the developeruses data loader to bulk insert the records.
below is the code snippet.

01 for(A__c a: trigger.new){
02 if(a.email__c != null){
03 List con = [select name from contact where email =: a.email__c];
04 If(con.size>0){
05 a.contactfound = true;
06 }
07 }
08 }

Which line of code is causing the issue?

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Which of the below requirements should be accomplished using the standard workflows instead of using process builder? (Select 2)

An Org has a custom object, each record of this custom object has a distinct record owner,
and the record owner is related to a parent account in Salesforce
Which relationship you should use to relate account to this custom object?

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What happens when a user update a record of this object?

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What is the order of execution events when we save a record in org?

How can developer match records to update existing records when loading data in an organization? (select 2)