According to the fourth paragraph, there was reason to worry when:

✂️ a) Ukraine raised oil prices from $20 a barrel to $60 in the last three years.

✂️ b) Ukraine refused to buy natural gas from Russia because of high prices.

✂️ c) Russia became a major supplier of oil and gas to the whole continent.

✂️ d) Russia stopped supplying natural gas to Ukraine in January 2006.

✂️ e) Europe asked Russia to cut natural gas supplies to Ukraine.

Responda: According to the fourth paragraph, there was reason to worry when: