In the following passage, some words have been deleted. Fill in the blanks with the help of the alternatives given. Select the most appropriate option for each blank.

Data 1. _______ the Pioneer spacecraft of NASA apparently prove the theory that the high surface temperature of Venus is 2. _______ to an atmospheric greenhouse effect caused mainly by a blanket of carbon dioxide. Such a greenhouse 3. _______ is created when energy in the form of sunlight easily passes through a planet's atmosphere, warms its 4. _______ and is converted to heat radiation that is then held in by the atmosphere from top to bottom. Venus has a relatively thin atmosphere like the Earth's, but Venus' atmosphere consists of more than ninety percent carbon dioxide, compared to less than four percent in that of the 5. _______.

Select the most appropriate option to fill in blank number 2.