71.
Your Answer is
Correct Answer is C
Explanation
According to the above, the tense is the simple present tense, and C is correct.
Item A/D: The subject of the sentence is the doorways and the verb should be plural.
Item B: It is an error-prone option. The latter sentence shows that "nails and wires are useless during construction". The past tense is used. This is an expression of the past, which is inconsistent with the previous time. expressed as objective facts.