Read the text below. Transform the words printed in capital letters at the end of the lines so that they grammatically and lexically correspond to the content of the text. Fill in the gaps with the correct words.
People spend hours talking on their mobile phones. There is an opinion that it may be __________________ \(HARM\) to their health, but it is difficult to know for sure.

• harmful
• harmless
• harmfull
• harmfully