Your exam technique
Identify the required information → locate evidence → fit the grammar → check the word limit.
Let the gap guide you
A gap after 'stored in' probably needs a place or container. Predict the grammatical role before locating the evidence. When instructions ask for words from the passage, use the relevant words from that passage and respect the maximum. An answer can express the right idea and still break the task's word limit.
Match the thing the question asks for
Matching headings asks for a paragraph's main idea; matching information may ask where a particular detail appears. In the latter task, a small example can be the decisive evidence. Read instructions about whether a paragraph letter or option can be reused, rather than assuming the rule is always the same.
Make a final accuracy pass
Read the completed sentence aloud in your head. Check that the selected words fit its grammar and meaning, and check spelling and number of words. Do not add an explanatory phrase simply to show you understood; extra words can make a valid answer invalid.
See it in practice
Starting point
Instruction: NO MORE THAN TWO WORDS. Passage: Samples were stored in sealed containers. Answer: in sealed containers
Using this skill
Answer: sealed containers
The question already supplies the preposition. Adding it both duplicates the grammar and exceeds the two-word limit.