You will be given a definition of a task first, then some input of the task.
In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: On the evening of 13 December Mawson and Mertz rearranged the sledges. The rear-most sledge, which had carried the most weight, was well-worn, and they decided to abandon it. The remaining supplies were re-distributed between the remaining two sledges. Most of the important supplies—the tent and most of the food—were stored on the new rear sledge; if they were to lose a sledge down a crevasse, they reasoned, it would be the front, less-vital sledge. As the rear sledge was heavier, the strongest of remaining dogs were assigned to pull it. At the camp they left a small amount of supplies, including the abandoned sledge and a tent cover, without the floor or poles.By noon the next day they had covered 311 miles (501 km) from the Cape Denison hut. Mertz was ahead on skis, breaking trail. Mawson sat on the first sledge; Ninnis walked beside the second. In his diary that night, Mertz recounted:
Around 1 pm, I crossed a crevasse, similar to the hundred previous ones we had passed during the last weeks. I cried out "crevasse!", moved at right angle, and went forward. Around five minutes later, I looked behind. Mawson was following, looking at his sledge in front of him. I couldn't see Ninnis, so I stopped to have a better look. Mawson turned round to know the reason I was looking behind me. He immediately jumped out of his sledge, and rushed back. When he nodded his head, I followed him, driving back his sledge.
Ninnis, his sledge and dog team had fallen through a crevasse 11 feet (3.4 m) wide with straight, ice walls. On a ledge deep in the hole, Mawson and Mertz could see the bodies of two dogs—one still alive, but seriously injured—and the remains of Ninnis' sledge. There was no sign of their companion. They measured the distance to the ledge as 150 feet (46 m), too far for their ropes to reach. "Dog ceased to moan shortly", wrote Mawson in his diary that night. "We called and sounded for three hours, then went a few miles to a hill and took position observations. Came back, called & sounded for an hour. Read the burial service.".
Output:
What is the name of the person who could not see Ninnis?