____the safety of gas, the government has checked the city’s gas supply system thoroughly.

A. To ensure           B. Ensuring        C. Having ensured        D. To have ensured

   the safety of gas, the government has checked the city’s gas supply system thoroughly.

________ the safety of gas, the government has checked the city’s gas supply system thoroughly．

1. A.
   To ensure
2. B.
   Ensuring
3. C.
   Having ensured
4. D.
   To have ensured