Data Sufficiency means you need to check whether the data given in the two statements is sufficient to answer the question asked or not. You need to find a unique answer to the question asked. More than one answer is not allowed.
First of all you need to read the directions of a particular Data Sufficiency question very carefully as examiner can change the directions and even after solving all the questions correctly you mark the wrong answers.
You need to remember the steps involved in solving a particular Data Sufficiency question and follow them in this particular order:
Check A (i.e. the first statement), then Check B (i.e. the second statement) and lastly, if required, combine the two statements to get the answer.
Do not make any assumptions while solving Data Sufficiency questions.