Virus Sniper Ltd (VSL) has recently developed a vaccine for the Coronavirus Disease 2019 (COVID-19) called "COVID Cleaner". After rigorous clinical tests, it is ready for release into the market. The company posted the following advertisement in the Hong Kong Morning Post:

COVID CLEANER

HK$10,000 reward will be paid to any person who purchases our vaccine "COVID CLEANER" but is still infected with the Coronavirus Disease 2019 after injection of two doses of the vaccine in one month by our company according to the printed instructions. As a sign of good faith, HK$1,000,000 has been deposited in the Millionaire Bank for security of the reward.

Having read the advertisement above, Mr. Fong purchased the vaccine "COVID Cleaner" from VSL and had received injection of two doses in one month by VSL according to the instructions. However, one day last month, Mr. Fong was feeling headache, tired and having a shortness of breath. He visited a medical doctor and was tested positive for the COVID-19. He was immediately sent to the hospital for quarantine and medical treatment. Receiving a treatment for 3 weeks, he had recovered completely. After being discharged fromthe hospital, Mr. Fong intended to claim VSL for the reward of HK$10,000.

Question: Explain whether the advertisement placed on the newspaper by VSL is an offer or an invitation to treat. You are required to cite a relevant case in your explanation