is / are confusion

[musicgold]

Hi,

Please refer to the following sentence. Please suggest whether I should use ‘is’ or ‘are’ at the underlined place.

‘Part of the reason behind this strength in property valuations is / are strong property fundamentals and the optimism about our economy. ‘

Thanks,

MG.

 

[Snowcake]

Please refer to the following sentence. Please suggest whether I should use ‘is’ or ‘are’ at the underlined place.

‘Part of the reason behind this strength in property valuations arestrong property fundamentals and the optimism about our economy. ‘