Jen Has Fair Skin, and Both Her Mother and Her

Jen has fair skin, and both her mother and her aunt have been treated for skin cancer. This coming weekend, Jen and her friends are planning an all-day outdoor hike near a high-elevation mountain lake; she expects there will be a mix of sun and shade on the trail and at the lakeshore beach. She is looking forward to the hike, but she wants to avoid getting a sunburn or any sun damage.
-Jen applies sunscreen to the exposed areas of her skin before she leaves her apartment for the hike;she should apply more sunscreen

A) 15 minutes after sun exposure begins.
B) 2 hours after sun exposure begins.
C) after climbing a hill and sweating heavily.
D) All of these choices are correct.