Which of the following factors may be regarded as the main cause the lanthanide contraction ?

Which of the following factors may be regarded as the main cause the lanthanide contraction ?
Which of the following factors may be regarded as the main cause the lanthanide contraction ?
Which of the following factors may be regarded as the main cause the lanthanide contraction ?
Which of the following factors may be regarded as the main cause the lanthanide contraction ?

contraction